During the day, if I wasn’t in the lab or studying, or out, it was my job to listen for the bell and answer the front door when it rang. This happened so infrequently that the first time it occurred it took me a minute to work out what the noise was.

It turned out to be Beverley Brook in an electric-blue quilted jacket with the hood up.

‘You took your time,’ she said. ‘It’s freezing out here.’

I said she should come in but she looked shifty and said she couldn’t.

‘Mum says I’m not to, she says that it’s inimical to the likes of us.’

‘Inimical?’

‘There’s, like, magic force fields and stuff,’ said Beverley.

That would make sense, I thought. It would certainly explain why Nightingale was so relaxed about security.

‘Why are you here, then?’

‘Well,’ said Beverley, ‘when a mummy river and a daddy river love each other very much …’

‘Funny.’

‘Mum says there’s some weird stuff at the UCH you should check out.’

‘What kind of weird stuff?’

‘She said it was on the news.’

‘We don’t have a TV,’ I said.

‘Not even Freeview?’

‘No kind of TV at all,’ I said.

‘Brutal,’ said Beverley. ‘You coming out, or what?’

‘I’ll go see what the Inspector says,’ I said.

I found Nightingale in the library making notes on what I strongly suspected was tomorrow’s Latin homework. I explained about Beverley, and he told me to check it out. By the time I got back to the lobby Beverley had risked coming just inside the door, although she stood as close to the threshold as she could get. Surprisingly Molly was standing beside her, their heads close together as if exchanging confidences. When they heard me coming they separated with suspicious speed – I felt my ears burning. Molly scurried past me and vanished into the depths of the Folly.

‘Are we taking the Jag?’ asked Beverley as I put my coat on.

‘Why, are you coming with me?’ I asked.

‘I have to,’ said Beverley. ‘Mum told me to facilitate.’

‘Facilitate what?’

‘The woman that called it in is an acolyte,’ said Beverley. ‘She won’t talk to you without me there.’

‘Okay,’ I said. ‘Let’s go.’

‘Are we taking the Jag?’

‘Don’t be stupid,’ I said. ‘UCH is walking distance.’

‘Aw,’ said Beverley. ‘I wanted to take the Jag.’

So we took the Jag and got caught in a traffic jam on the Euston Road, and then spent another twenty minutes looking for a parking space. It took us, I estimated, twice as long to drive as it would have to walk.

University College Hospital takes up two whole blocks between Tottenham Court Road and Gower Street. Founded in the nineteenth century, its main claim to fame was as the teaching hospital for the University College of London and the birthplace of one Peter Grant, apprentice wizard. Since that momentous day in the mid-1980s, half the site had been redeveloped into a gleaming blue and white tower that looked as if a bit of Brasilia had crash-landed in the middle of Victorian London.

The lobby was a wide, clean space with lots of glass and white paint marred only by the large numbers of sick people shuffling around. Police officers spend a lot of time in A&E, since you’re either asking people where they got their knife wounds, dealing with the violently drunk or being stitched up yourself. It’s one of the reasons so many coppers marry nurses – that, and the fact that nurses understand about unreasonable shift systems.

Beverley’s acolyte was a nurse, a pale skinny one with purple hair and an Australian accent. She stared at me suspiciously.

‘Who’s this?’ she asked Beverley.

‘This is a friend,’ Beverley said and put her hand on the woman’s arm. ‘We tell him everything.’

The woman relaxed and gave me a smile full of hope. She looked like one of the Pentecostal teenagers from my mum’s second-from-last church. ‘Isn’t it wonderful to be part of something real?’ she said.

I agreed that being part of something real was indeed wonderful, but it would be groovy if she could tell me what she’d seen. I actually used the word ‘groovy’ and she didn’t even flinch, which was worrying on so many levels.

According to her, a cycle courier had been brought in by ambulance following a road traffic accident, and while he was being treated he’d kicked the attending doctor in the eye. The doctor had been stunned rather than seriously injured and the cycle courier had run out of A&E before Security could nab him.

‘Why bring it to us?’ I asked.

‘It was the laughing,’ said the nurse. ‘I was going back to the treatment bay when I heard this screeching laugh, like a mynah bird. Then I heard Eric – Dr Framline, that’s the doctor who was injured – I heard him swearing, and then the cycle courier comes charging out of the bay and there’s something wrong with his face.’

‘Wrong, how?’ I asked.

‘Just wrong,’ she said, displaying precisely the characteristic that makes eyewitnesses such a useful part of any police investigation. ‘He went past so fast I didn’t see much but it just looked … wrong.’

She showed me the treatment bay where it happened, a white and beige cubicle with an examination bed and a curtain for privacy. The vestigium – note that I’m using the singular here – slapped me in the face as soon as I walked in. Violence, laughter, dried sweat and leather. It was the same as poor William Skirmish when he was lying in the mortuary, only minus the annoying yappy dog.

Two months previously I would have walked into that treatment bay, shivered, thought, ‘That’s weird’ and walked right back out again.

Beverley stuck her head in and demanded to know whether I’d found anything.

‘I need to borrow your phone,’ I said.

‘What happened to yours?’ she asked.

‘I blew it up in a magic accident,’ I said. ‘Don’t ask.’

Beverley pouted and handed over a surprisingly chunky Ericsson. ‘You have to top it up,’ she said. The casing had latex seals and the buttons were large and protected by a layer of clear plastic. ‘It’s designed to go underwater,’ she said. ‘Don’t ask.’

‘Can you get your acolyte to find out Dr Framline’s address for me?’

Beverley shrugged. ‘Sure,’ she said. ‘And remember, you talk, you pay!’

While Beverley was distracted with her task I took her phone outside to Beaumont Place, a quiet pedestrianised road that ran between the old and the new bits of the hospital, and called Nightingale. I described the incident and the vestigium and he agreed that it was worth stepping up the search for the courier.

‘I want to keep an eye on the doctor,’ I said.

‘Interesting,’ said Nightingale. ‘Why?’

‘I’m thinking of the sequence of events around Skirmish’s murder,’ I said. ‘Toby bites Coopertown on the nose, that’s when it starts. But Coopertown doesn’t go postal until later when he runs into Skirmish in Covent Garden.’

‘You think it was set off by a chance meeting?’

‘That’s just it,’ I said. ‘Lesley says that the Murder Team haven’t found a reason for Skirmish even to be in Covent Garden that night. He gets a bus down to the West End, meets Coopertown and gets his head knocked off. No meetings, no friends – nothing.’

‘You think both parties were affected?’ asked Nightingale. ‘You think an outside agency made them meet?’

‘Is such a thing possible?’

‘Anything’s possible,’ said Nightingale. ‘If your dog was affected along with his master and Coopertown, then it would explain why he was so sensitive to the vestigia.’

I noticed Toby was my dog now. ‘So it’s possible?’

‘Yes,’ said Nightingale, but I could tell he was sceptical.

‘What if the cycle courier is playing Toby’s role and the doctor is taking Coopertown’s?’ I asked. ‘At the very least it wouldn’t hurt to keep an obbo on the doctor until the courier is caught.’

‘Can you handle that?’ asked Nightingale.

‘No problem,’ I said.

‘Good,’ said Nightingale, and offered to coordinate the search for the cycle courier. I hung up as Beverley Brook sauntered over from the hospital, the swing of her hips dragging at my eyes. She grinned when she caught me looking and handed me a slip of paper – Dr Framline’s address.

‘What next, guv?’ she asked.

‘Where can I drop you?’ I asked.

‘No, no, no,’ Beverley said quickly. ‘Mum says I was to facilitate.’

‘You’ve facilitated,’ I said. ‘You can go home now.’

‘I don’t want to go home,’ she said. ‘Mum’s got the whole entourage round, Ty and Effra and Fleet, not to mention all the old ladies. You don’t know what it’s like.’

Actually I knew exactly what it was like, but I wasn’t going to tell Beverley that.

‘Come on, I’ll be good,’ she said, giving me the big eyes. ‘I’ll let you borrow my phone.’

I gave in before she escalated to the trembling lip. ‘But you have to do what I say.’

‘Yes guv,’ she said, and saluted.

You can’t do an obbo in a vintage Jag, so, much to Beverley’s disappointment, we drove back to the Folly to swap it for the ex-Panda. The Folly’s garage is out the back of the building and takes up the entire bottom floor of the converted coach house. From the mews you can see where the original doors, wide and high enough to accept a coach and four, had been bricked in and replaced with a more modest sliding door. The Jag and the ex-Panda rattled around inside a space big enough for four carriages.

Unlike the entrance hall, the coach house didn’t seem to bother Beverley at all. ‘What happened to the inimical magic force fields?’ I asked.

‘Not in here,’ she said. ‘Bit of protection on the garage door and that’s it.’

Nightingale had left the building, but Molly met me in the lobby with a Tesco’s carrier bag full of sandwiches wrapped in greaseproof paper and tied up with string. I didn’t ask what was in them but I doubted it was chicken tikka masala. Back in the coach house, I threw my bag and sandwiches in the back of the ex-Panda, made sure Beverley had her seat belt on and headed off to harass a junior doctor.

Dr Framline lived in a two-storey Victorian terrace off the Romford Road in Newham. It was further east than I like to go but not a bad neighbourhood. I found a parking space with decent sightlines of the front door and got out – I knew no force on earth was going to keep Beverley in the car, so I let her come with me on the strict understanding that she’d keep her mouth shut.

There was only one doorbell and the small front garden was given over to gravel, the dustbins and a couple of empty, bright red plant pots. I was thinking that either Dr Framline owned the whole place or he was sharing with friends. I pressed the bell and a cheerful voice said it was on its way. The voice belonged to a plump, round-faced woman of the sort that develops a good personality because the alternative is suicide.

I showed her my warrant card. ‘Good afternoon. My name’s Peter Grant, I’m from the police and this is my colleague Beverley Brook, who’s a river in south London.’ You can get away with stuff like that with civilians because their brains lock in place on the word ‘police’.

Actually I think I may have overdone it because the woman frowned at Beverley and asked, ‘Did you just say she was a river?’ Which is why you should never show off when on duty.

‘It’s an office joke,’ I said.

‘She seems a bit young to be police,’ said the woman.

‘She’s not,’ I said. ‘She’s on work experience.’

‘Can I see your identification again?’ asked the woman.

I sighed and handed it over. Beverley sniggered.

‘I can give you the number of my superior if you like,’ I said. This normally does the trick, since members of the public are generally lazier than they are suspicious.

‘Are you here about what happened at the hospital?’ asked the woman.

‘Yes,’ I said, relieved. ‘That’s exactly why we’re here.’

‘Only Eric’s gone into town,’ she said. ‘You just missed him; he went fifteen minutes ago.’

Of course he has, I thought, no doubt to some spot less then five hundred metres from where Beverley and I started out. ‘Do you know where he was going?’

‘Why do you want to know?’

‘We think we have a line on the man who attacked him,’ I said. ‘We just need him to confirm a few details. If we do this quickly we may be able to make an arrest tonight.’

That perked her up, and got me not just the name of the gastropub Dr Framline was heading for but also his mobile number. Beverley had to trot to keep up with me as we headed back to the car.

‘What’s the rush?’ she asked as we climbed in.

‘I know the pub,’ I said. ‘It’s on the corner of Neal Street and Shelton Street.’ I pulled out without waiting for Beverley to buckle up. ‘Right across from there is the pedestrian space outside Urban Outfitters.’

‘Urban Outfitters, eh,’ said Beverley. ‘That explains the Dr Denim shirt.’

‘My mum bought me that,’ I said.

‘And you think that’s less embarrassing?’

I gunned the ex-Panda, or at least I came as close to gunning it as you can with a ten-year-old Ford Escort, and went through a set of lights on red. There was a yell behind me. ‘Cycle couriers like to hang out there,’ I said. ‘It’s convenient for the pub and cafés, but also close to most of their clients.’

Rain began to splatter on the windscreen and I had to ease up – the streets were getting wet. How long would it take Dr Framline to reach Covent Garden by public transport? Not less than an hour, but he had a head start and this was London, where the tube was often faster than the car.

‘Call Dr Framline,’ I told Beverley.

She grumbled, dialled, listened and said, ‘Voice mail. He’s probably underground.’

I gave her Lesley’s number. ‘Remember,’ she said, ‘you talk, you pay.’

‘That’s the way it works,’ I said.

Beverley held the phone to my ear so I could keep both hands on the controls. When Lesley picked up I could hear the incident room at Belgravia in the background – proper police work.

‘What happened to your phone?’ she asked. ‘I’ve been trying to ring you all morning.’

‘I broke it doing magic,’ I said. ‘Which reminds me: I need you to book me out an Airwave.’ Airwave was the all-singing, all-dancing digital radio handset for coppers.

‘Can’t you get one from your nick?’ she asked.

‘You’re joking,’ I said. ‘I don’t think Nightingale’s got the hang of Airwave yet. Or even radios, for that matter. In fact, I think he might be a bit hazy on telephones.’

She agreed to meet us at Neal Street.

The rain was sheeting down as I crawled up the semi-pedestrianised length of Earlham Street and stopped on the corner, where we could get a good view of the pub and the cycle-courier hangout. I left Beverley in the car and popped across to check inside the pub. It was deserted; Dr Framline hadn’t arrived yet.

My hair was soaked through when I got back in the car but I had a towel in my obbo bag, and I used it to squeeze most of the water out. For some reason Beverley found this hilarious.

‘Let me do that,’ she said.

I handed her the towel and she leaned over and started rubbing my head. One of her breasts pushed against my shoulder and I had to resist an urge to put my arm around her waist. She dug her fingers into my scalp.

‘Don’t you ever comb this?’ she asked.

‘I can’t be bothered,’ I said. ‘I just shave it down to stubble every spring.’

She ran her palm over my head and let it rest, lightly, on the back of my neck. I felt her breath close, on my ear.

‘You really got nothing from your dad, did you?’ Beverley sat back in her own seat and tossed the towel into the back. ‘Your mum must have been disappointed. I bet she thought you’d have big curls.’

‘It could have been worse,’ I said. ‘I could have been a girl.’

Beverley unconsciously touched her own hair, which was straightened and side-parted into wings that reached to her shoulders. ‘You don’t know the half of it,’ she said. ‘Which is why you ain’t going to get me out into that.’ She nodded at the rainswept streets.

‘If you’re so supposed to be a goddess …’

‘Orisa,’ said Beverley. ‘We’re Orisa. Not spirits, not local geniuses – Orisa.’

‘Why can’t you do something about the weather?’ I asked.

‘For a start,’ she said with exaggerated slowness, ‘you don’t mess with the weather, and second, this is north London and this manor belongs to my older sisters.’

I’d found a seventeenth-century map of the rivers of London. ‘That would be the Fleet and the Tyburn?’ I asked.

‘You can call her Tyburn if you want to spend the rest of the day dangling from a noose,’ said Beverley. ‘If you ever meet her, you better make sure you call her Lady Ty. Not that you ever want to meet her. Not that she ever wants to meet you.’

‘So you don’t get on with them?’ I asked.

‘Fleet is okay,’ she said. ‘But nosy. Ty is just stuck up. She lives in Mayfair and goes to posh people’s parties and knows “people that matter”.’

‘Mum’s favourite?’

‘Only because she fixes stuff with the politicians,’ said Beverley. ‘Has tea on the terrace at the Palace of Westminster. I get to sit in a car with Nightingale’s errand boy.’

‘If I remember, you’re the one who didn’t want to go home,’ I said.

I spotted Lesley’s car pulling up behind us. She flashed her lights and got out. I quickly leaned back to open the passenger door for her. Rain hit me in the face hard enough to make me splutter, and Lesley practically threw herself onto the back seat.

‘I think it’s going to flood,’ she said and seized my towel, using it to dry her face and hair. She jerked her head at Beverley. ‘Who’s this?’ she asked.

‘Beverley, this is PC Lesley May.’ I turned to Lesley. ‘This is Beverley Brook, river spirit and winner of the London Regional All-comers Continuous Talking Championship five years running.’ Beverley punched me in the arm. Lesley gave her an encouraging smile. ‘Her mother is the Thames, you know.’

‘Really,’ said Lesley. ‘Who’s your dad, then?’

‘That’s complicated,’ said Beverley. ‘Mum said she found me floating down the brook by the Kingston Vale dual carriageway.’

‘In a basket?’ asked Lesley.

‘No, just floating,’ said Beverley.

‘She was spontaneously created by the midichlorians,’ I said. Both women gave me blank looks. ‘Never mind.’

‘Has your man arrived yet?’ asked Lesley.

‘Nobody’s arrived since we got here,’ I said.

‘Do you know what he looks like?’ said Lesley.

I realised that I didn’t have the faintest idea what Dr Framline looked like. I’d been expecting to interview him at home before I followed him. ‘I have a description,’ I said. Lesley gave me a pitying look and pulled out A4 hard copy of the photo from Dr Framline’s driving licence. ‘He’d be a decent copper,’ she told Beverley, ‘if he could just keep his mind on the details.’

She handed me something that looked like the chunky mutant offspring of a Nokia and a walkie-talkie – an Airwave handset. I stuffed it in the inside pocket of my jacket. The handset is a bit heavier than a mobile phone, and was going to make me lopsided.

‘Is that him?’ asked Beverley.

We peered out into the rain and saw a couple approaching from the Covent Garden end of Neal Street. The man’s face matched the photograph apart from the bruising around his left eye and the railway track of adhesive strips holding the cut on his cheek together. He held an umbrella over himself and his companion, a stocky woman in a lurid orange waterproof. They were both smiling and seemed happy.

We watched in silence as they reached the gastropub and, with a pause to shake out his umbrella, went inside.

‘Remind me why we’re here again?’ asked Lesley.

‘Have you found the cycle courier yet?’ I asked.

‘No,’ said Lesley. ‘And I don’t think my governor likes your governor treating him as his errand boy.’

‘Tell him, welcome to the club,’ I said.

‘You tell him,’ said Lesley.

‘So what’s in the sandwiches?’ asked Beverley.

I opened the Tesco’s bag and unwrapped the packets to find crusty white bread filled with roast beef and mustard pickle garnished with horseradish – very nice, but once my packed lunch had been fried calves’ brain, so I tended to approach Molly’s sandwiches with caution. Lesley, who eats without fear and thinks eels in jelly are a delicacy, dived in but Beverley hesitated.

‘If I eat these, you’re not going to expect an obligation, are you?’ asked Beverley.

‘Don’t worry about it,’ I said. ‘I have an air freshener in the bag.’

‘I’m serious,’ said Beverley. ‘There’s a geezer at my mum’s flats who turned up to repossess some furniture in 1997. One cup of tea and a biscuit later, and he’s never left. I used to call him Uncle Bailiff. He does odd jobs around the place, fixes stuff and keeps the place clean and my mother will never let him go.’ Beverley jabbed me in the chest with her finger. ‘So I want to know what your intentions are with this sandwich.’

‘I assure you, my intentions are honourable,’ I said, but part of me was thinking about how close I came to eating that custard cream back at Mama Thames’s flat.

‘Swear it on your power,’ said Beverley.

‘I don’t have any power,’ I said.

‘Good point,’ said Beverley. ‘Swear it on your mum’s life.’

‘No,’ I said. ‘This is childish.’

‘Fine,’ said Beverley. ‘I’ll get my own food.’ She got out of the car and stomped away, leaving the door open. I noticed that she’d waited for the rain to ease up before throwing a fit.

‘Is that true?’ asked Lesley.

‘Which bit?’ I asked.

‘Spells, food, obligations, wizards – the bailiff,’ said Lesley. ‘For God’s sake, Peter, that’s false imprisonment at the very least.’

‘Some of it’s true,’ I said. ‘I don’t know how much. I think becoming a wizard is about discovering what’s real and what isn’t.’

‘Is her mum really the goddess of the Thames?’

‘She thinks she is, and I’ve met her and I’m beginning to think she might be,’ I said. ‘She’s got real power, so I’m going to treat her daughter as the real thing until I find out different.’

Lesley leaned over the seat back and looked me in the eyes.

‘Can you do magic?’ she asked softly.

‘I can do one spell,’ I said.

‘Show me.’

‘I can’t,’ I said. ‘If I do it now I’ll blow the Airwave sets, the stereo and possibly the ignition system. That’s how I busted my phone – I had it in my pocket when I was doing my practice.’

Lesley tilted her head to the side and gave me a cool look.

I was about to protest when Beverley banged on my window – I rolled it down.

‘I just thought you ought to know that it’s stopped raining,’ she said. ‘And there’s a cycle courier walking down the street.’

Me and Lesley piled out of the car, which shows how inexperienced we really were at basic surveillance, remembered that we were trying to be unobtrusive and pretended to be having a casual chat with each other. In our defence we’d just spent two years in uniform, and being obtrusive is what a uniformed constable is all about.

Beverley must have had good eyes because the courier was at the Shaftesbury Avenue end of Neal Street and was approaching at a slow, deliberate pace. He was pushing his bike, which was suspicious, and I saw that the back wheel was bent out of shape. I felt a deep sense of unease, but I couldn’t tell if that was me or something external.

In the near distance a dog started barking. Behind us, a mother told off a child who wanted to be carried. I could hear rain draining into a gutter somewhere and I found myself straining to hear – I’m not sure what. Then I heard it: a thin, strangled, high-pitched giggle that seemed to float in from far away.

The cycle courier looked normal enough, dressed in painfully tight yellow and black Lycra, a messenger bag with a radio attached to its shoulder strap and a street helmet in blue and white. He had a narrow face and a mouth that was a thin line under a sharp nose, but his eyes were worryingly blank. I didn’t like the way he was walking. The twisted back wheel was scraping the forks and the man’s head seemed to bob unnaturally on his neck in time with every revolution. I decided it would be a bad idea to let him get any closer.

‘Bastard!’ There was a shout behind me and a rattling crash.

I turned and saw nothing until Lesley pointed to the glass double doors of Urban Outfitters. A man was being slammed violently against the inside of the doors. He was jerked out of sight and then smashed against the doors again – hard enough to pop one of the hinges and make a gap large enough for the man to escape. He looked like a tourist or foreign student, well dressed in the European style – dirty blond hair cut the respectable side of too long, a blue Swissair complimentary knapsack still hooked over one shoulder. He shook his head as if bewildered, and flinched back as his attacker smacked open the doors and strode towards him.

This was a short, plump man with thinning brown hair and round, wire-framed glasses. He was wearing a white shirt with a manager’s tag clipped to the pocket. He was sweating and his shining face was red with rage.

‘I’ve fucking had it,’ he screamed. ‘I try to be polite, but no, you’ve got to fucking treat me like I’m some fucking slave.’

‘Oi,’ shouted Lesley, ‘police.’ She advanced on them, warrant card in her left hand, her right hand resting on the handle of her extendable baton. ‘What seems to be the problem?’

‘He attacked me,’ said the young man. Definitely an accent. German, I thought.

The enraged shop manager hesitated and turned to look at Lesley, his eyes blinking behind his specs. ‘He was talking on the phone,’ said the manager. The violence seemed to have drained out of him. ‘While he was at the till. It’s not even like he got a call – he dialled it himself while he was paying. I’m expected to have a mutually beneficial and courteous interaction with him, and the fucker ignores me and makes a phone call.’

Lesley stepped between the two men and gently edged the manager backwards. ‘Why we don’t go inside,’ she said, ‘and you can tell me all about it.’ It really was a delight to watch her work.

‘I mean, why?’ said the manager. ‘What was so important it couldn’t wait?’

Beverley smacked me in the arm. ‘Peter,’ she said. ‘Over there.’

I turned just in time to see Dr Framline charge up the street brandishing a stick half as tall as he was. Behind him came his date from the gastropub, yelling his name in confusion. I ran as fast as I could, passing the woman quickly, but there was no way I could get to Dr Framline before he reached his target.

The courier didn’t even put an arm up to defend himself when Dr Framline clubbed him hard on the shoulder with the stick. I saw the man’s right arm jerk brokenly and his hand lose its grip on the bike, which began to topple sideways.

‘The more you take,’ yelled the doctor, raising the stick again, ‘the better it is for you.’

I hit him low, getting my shoulder into the sweet spot just above his hips so that he went sideways and down and broke my fall instead of the other way round. I heard the bike hit the street and then the stick skittering across the pavement. I tried to pin Dr Framline but he seemed amazingly strong, and jammed an elbow into my chest hard enough to leave me gasping for breath. I made a grab for his legs and got a knee in the face that made me swear.

‘Police,’ I shouted. ‘Stop fighting.’ Amazingly, he did. ‘Thank you,’ I said; it seemed only polite. I tried to get up but somebody fetched me such a blow that I was face down on the pavement again before it even registered I’d been hit. In a street fight, no matter how hurt you are the pavement is not your friend, so I rolled over and tried to get back up again. As I did I saw the cycle courier grab the outsized stick off the ground and swing at Dr Framline. The doctor flinched out of the way but the stick caught him on the upper part of his arm. He slipped over and went down, gasping in pain.

A wave of emotion washed over me: elation, excitement and an undertone of violence, like that of the home crowd at a football match when their team gets a chance at the goal.

I saw the dissimulo as it happened that time: the courier’s chin seemed to bulge, I heard the distinct cracking of bone and teeth as it jutted forward into a sharp point. The lips twisted into a snarl as the nose stretched until it was almost as long. It wasn’t a real face, it was a caricature man-in-the-moon face that no human could have in real life. The mouth opened and I could see inside to the red ruin of his jaw.

‘That’s the way to do it!’ he shrieked and lifted his stick.

Lesley’s baton hit him in the back of the head. He staggered, Lesley hit him again and with a gurgling sigh he fell forward in front of me. I crawled over and rolled him on his back but it was too late. His face slumped like wet papier mâché. I saw the skin tearing around the nose and chin and then a great dripping flap peeled open and lolled over his forehead. I tried to make myself do something, but nothing in my first-aid training had prepared me for someone’s face flopping open like a starfish.

I slid my palm under the flap of skin, flinching at the warm wetness, and tried to fold it back over the face. I had some vague idea that I should at least try and stop the bleeding.

‘Let me go,’ yelled Dr Framline. I looked over and saw that Lesley already had him in handcuffs. ‘Let me go,’ he said. ‘I can help him.’ Lesley hesitated.

‘Lesley,’ I said, and she started uncuffing the doctor.

Too late. The courier went suddenly rigid, his back arched and a tide of blood welled up from his neck and forced itself out through the rips in his skin and the gaps between my fingers.

Dr Framline scrambled over and jammed his finger into the courier’s neck. He shifted their position, looking for a pulse, but I could see in his expression that there was none. Finally he shook his head and told me to let go. The courier’s face flopped open again.

Somebody was screaming and I had to check it wasn’t me. It could have been me. I certainly wanted to scream, but I remembered that, right then and there, Lesley and I were the only coppers on the scene, and the public doesn’t like it when the police start screaming: it contributes to an impression of things not being conducive to public calm. I got to my feet and found that we’d attracted a crowd of onlookers.

‘Ladies and gentlemen,’ I said, ‘police business. I need you to stand back.’

The crowd stood back – being covered in blood can have that effect on people.

We preserved the scene until back-up arrived, but two-thirds of the crowd had their phones out and were taking video and stills of me, Lesley and the mutilated remains of the cycle courier. The images had already hit the internet before the ambulance arrived and the paramedic had covered the poor sod with a sheet. I spotted Beverley hanging around near the back of the crowd and when she saw that, she caught my eye, gave me a little wave, turned and walked away.

Me and Lesley found a place under a shop awning and waited for the forensic tent, the swabs and the replacement bunny suit.

‘We can’t keep doing this,’ said Lesley. ‘I’m running out of clothes.’

We laughed – sort of. It’s not that it gets easier the second time, it’s just that by then you know you’re still going to wake up the next morning the same person who went to sleep.

A DS from the Murder Team arrived and took charge. She was a squat, angry-faced middle-aged woman with lank brown hair who looked like she fought Rottweilers for a hobby. This was the legendary Detective Sergeant Miriam Stephanopoulos, Seawoll’s right-hand woman and terrifying lesbian. The only joke ever made at her expense goes: ‘Do you know what happened to the last police officer who made a joke about DS Stephanopoulos?’ ‘No, what did happen to him?’ ‘Nobody else knows, either.’ I said it was the only joke, not a good one.

She seemed to have a soft spot for Lesley, though, so we got processed much faster this time, but as soon as we were done we were bundled into an unmarked car and driven to Belgravia. Nightingale and Seawoll debriefed us in an anonymous conference room at which nobody took notes, but at least we were offered tea.

Seawoll glared at Lesley; he wasn’t happy. Lesley glared at me; she wasn’t happy that Seawoll wasn’t happy. Nightingale wasn’t anything except distracted; he only seemed interested when I reported my sense impressions just prior to the attack. After the briefing we trooped over to the Westminster mortuary, where surprisingly both Seawoll and Stephanopoulos attended the autopsy. Lesley and I made a point of standing behind them in the hope they wouldn’t notice us.

The cycle courier lay on the table with his face splayed open in a way that was becoming horribly familiar. Dr Walid was giving his conclusion that, somehow, person or persons unknown had managed to trick the victim into changing his face with magic and then set him to attacking random strangers. DS Stephanopoulos gave Seawoll a sharp look at the word magic, but her boss gave a small shake of his head that said, Later, not here.

‘His name was Derek Shampwell,’ said Dr Walid. ‘Age twenty-three, Australian citizen, had been in London for three years, no criminal record, hair analysis shows intermittent marijuana use over the last two years.’

‘Do we know why he was singled out?’ asked Seawoll.

‘No,’ said Nightingale. ‘Although all the cases seem to start with a sense of grievance. Coopertown was bitten by someone’s pet, Shampwell was struck by a motor vehicle while riding.’

Seawoll glanced at Stephanopoulos. ‘Hit and run on the Strand, sir, in a CCTV blind spot.’

‘A blind spot?’ asked Seawoll. ‘On the Strand?’

‘Thousand-to-one chance,’ said Stephanopoulos.

‘May,’ barked Seawoll, without turning round. ‘You think there are related cases?’

‘Including the incident Grant and I witnessed in the cinema, and the one that took place just prior to Shampwell’s death, I’ve identified fifteen cases where the perpetrators have shown uncharacteristic levels of aggression,’ said Lesley. ‘All people with clean records, no psychiatric history and all within half a mile of Cambridge Circus.’

‘How many do we know were actually’ – Seawoll paused – ‘possessed?’

‘Just the ones whose faces fell off,’ said Nightingale.

‘Just so we’re clear,’ said Seawoll. ‘The Commissioner wants this kept quiet, so PC May liaises with PC Grant for the low-level stuff but anything significant, anything at all, you talk to me. Do you have a problem with this, Thomas?’

‘Not at all, Alexander,’ said Nightingale. ‘It all seems eminently sensible.’

‘His parents are flying in tomorrow,’ said Dr Walid. ‘Is it all right if I sew his face back together?’

Seawoll glared at the body. ‘Fuck,’ he said.

Nightingale was silent on the drive back to the Folly, but at the foot of the stairs he turned to me and told me to get a good night’s sleep. I asked him what he was going to do, and he said he’d work on some research in the library – see if he couldn’t narrow down what was doing the killing. I asked if I could help.

‘Train harder,’ he said. ‘Learn faster.’

As I went upstairs, I met Molly gliding down. She paused and gave me an inquiring look.

‘How should I know?’ I said. ‘You know him better than I do.’

You don’t tell your governor that you need a broadband connection, cable for preference, because you want to watch football. You tell him that you need the internet so you can access HOLMES directly instead of having constantly to rely on Lesley May. The football coverage, movies on demand and multiplayer console games are all merely serendipitous extras.

‘Would this involve physically running a cable into the Folly?’ asked Nightingale when I tackled him during practice in the lab.

‘That’s why they call it cable,’ I said.

‘Left hand,’ said Nightingale, and I dutifully produced a werelight with my left hand.

‘Sustain it,’ said Nightingale. ‘We can’t have anything physically entering the building.’

I’d got to the point where I could talk while sustaining a werelight, although it was a strain to make it look as casual as I did. ‘Why not?’

‘There’s a series of protections woven around the building,’ said Nightingale. ‘They were last set up after the new phone lines were put in in 1941. If we introduce a new physical connection with the outside, it would create a weak spot.’

I stopped trying to be casual and concentrated on maintaining the werelight. It was a relief when Nightingale told me to stop.

‘Good,’ he said. ‘I think you’re almost ready to move on to the next form.’

I dropped the werelight and caught my breath. Nightingale wandered over to the adjoining bench, where I’d dismantled my old mobile phone and set up the microscope I’d found in a mahogany case in one of the storage cupboards.

He touched the brass and black-lacquer tube. ‘Do you know what this is?’ he asked.

‘An original Charles Perry number 5 microscope,’ I said. ‘I looked it up on the internet. Made in 1932.’ Nightingale nodded, and bent down to examine the insides of my phone.

‘You think magic did this?’ he asked.

‘I know it was the magic,’ I said. ‘I just don’t know how or why.’

Nightingale shifted uncomfortably. ‘Peter,’ he said. ‘You’re not the first apprentice with an inquiring mind, but I don’t want this getting in the way of your duties.’

‘Yes sir,’ I said. ‘I’ll keep it to my free time.’

‘You’re about to suggest the coach house,’ said Nightingale.

‘Sir?’

‘For this cable connection,’ said Nightingale. ‘The heavy defences tended to disturb the horses, so they skirt the coach house. I’m sure this cable connection of yours will be very useful.’

‘Yes sir.’

‘For all manner of entertainments,’ continued Nightingale.

‘Sir.’

‘Now,’ said Nightingale. ‘The next form – Impello.’

I couldn’t tell whether the coach house had originally been built with a first floor, to house footmen or whatever, which had then been knocked through in the 1920s, or whether the floor had been added by sticking a new ceiling on the garage when they bricked up the main gate. At some point someone had bolted a rather beautiful wrought-iron spiral staircase to the courtyard wall. When I’d first ventured up, I was surprised to find that a good third of the sloping roof on the south-facing side had been glazed. The glass was dirty on the outside and some of the panes were cracked, but it let in enough daylight to reveal a jumble of shapes shrouded by dust sheets. Unlike those in the rest of the Folly, these sheets were furry with dust – I didn’t think Molly had ever cleaned in there.

If the chaise longue, Chinese screen, mismatched side tables and collection of ceramic fruit bowls that I found under the sheets weren’t enough of a clue, I also found an easel and a box full of squirrel-hair paint brushes gone rigid with disuse. Somebody had used the rooms as a studio, judging from the empty beer bottles neatly lined up against the south wall. Probably apprentices like me – that, or a wizard with a serious alcohol problem.

Stacked in the corner and carefully wrapped in brown paper and string were a series of canvases, painted in oils. These included a number of still lives, a rather amateurish portrait of a young woman whose discomfort was palpable, despite the sloppy execution. The next was much more professional – an Edwardian gentleman reclining in the same wickerwork chair I’d found under a dust sheet earlier. The man was holding a silver-topped cane and for a moment I thought he might be Nightingale, but the man was older and his eyes were an intense blue. Nightingale senior, perhaps? The next, probably by the same painter, was a nude with a subject that so shocked me I took it to the skylight to get a better look. I hadn’t made a mistake. There was Molly, reclining pale and naked on the chaise longue, staring out of the canvas with heavy-lidded eyes, one hand dipping into a bowl of cherries placed on a table by her side. At least, I hope they were cherries. The painting was in the impressionist style so the brush strokes were bold, making it hard to tell: they were definitely small and red, the same colour as Molly’s lips.

I carefully rewrapped the paintings and put them back where I’d found them. I did a cursory check of the room for damp, dry rot and whatever it is that makes wooden beams crumbly and dangerous. I found that there was still a shuttered loading door at the courtyard end of the room and mounted above it, a hoisting beam. Presumably to serve a hayloft for the coach horses.

As I leaned out to check it was still solid, I saw Molly’s pale face in one of the upper windows. I didn’t know what I found stranger, that somebody had persuaded her to get her kit off, or that she hadn’t changed in appearance in the last seventy years. She withdrew without apparently seeing me. I turned and looked around the room.

This, I thought, will do nicely.

At one time or other most of my mum’s relatives had cleaned offices for a living. For a certain generation of African immigrants cleaning offices became part of the culture like male circumcision and supporting Arsenal. My mum had done a stint herself and had often taken me with her to save on babysitting. When an African mum takes her son to work she expects her son to work so I quickly learnt how to handle a broom and a window cloth. So the next day after practice, I returned to the coach house with a packet of Marigold gloves and my Uncle Tito’s Numatic vacuum cleaner. Let me tell you, 1,000 watts of suckage makes a big difference when cleaning a room. The only thing I had to worry about was causing a rift in the space-time fabric of the universe. I found the window cleaners online, and a pair of bickering Romanians scrubbed up the skylight while I rigged up a pulley to the hoisting beam, just in time for the TV to be delivered along with the fridge.

I had to wait a week for the cable to be hooked up, so I caught up on my practice and started narrowing down the location of Father Thames. ‘Finding him will be a good exercise for you,’ Nightingale had said. ‘Give you a good grounding in the folklore of the Thames Valley.’ I asked for a clue, and he told me to remember that Father Thames had traditionally been a peripatetic spirit which, according to Google, meant walking or travelling about, itinerant, so not really a lot of help. I had to admit that it was expanding my knowledge of the folklore of the Thames Valley, most of which was contradictory but would no doubt be helpful at the next pub quiz I took part in.

To inaugurate my re-entry into the twenty-first century I ordered some pizza and invited Lesley round to see my etchings. I had a long soak in the claw-footed porcelain tub that dominated the communal bathroom on my floor and swore, not for the first time, that I was definitely going to install a shower. I’m not a peacock but on occasion I like to dress to impress, although like most coppers I don’t wear much in the way of bling, the rule being never wear something round your neck that you don’t want to be strangled with. I laid in some Becks because I knew Lesley preferred bottled beer, and settled in to watch sports TV while I waited for her to turn up.

Among the many other modern innovations that I’d introduced to the coach house was an entryphone installed on the garage’s side door, so that when Lesley arrived all I had to do was buzz her in.

I opened the door and met her at the top of the spiral staircase – she’d brought company.

‘I brought Beverley,’ said Lesley.

‘Of course you did,’ I said.

I offered them beer. ‘I want you to make it clear that nothing I eat or drink here puts me under obligation,’ said Beverley. ‘And no mucking me about this time.’

‘Fine,’ I said. ‘Eat, drink, no obligations, Scout’s honour.’

‘On your power,’ said Beverley.

‘I swear on my power,’ I said.

Beverley grabbed a beer, hopped onto the sofa, found the remote and started channel-surfing. ‘Can I on-demand a movie?’ she asked. There followed a three-way argument over what we were going to watch, which I lost at the start and Lesley won in the end by the simple expedient of grabbing the remote and switching to one of the free movie channels.

Beverley was just complaining that none of the pizzas had pepperoni when the door opened a fraction and a pale face peered in. It was Molly. She stared at us, and we stared back.

‘Would you like to come in?’ I asked.

Molly slipped silently inside and drifted over to the sofa, where she sat next to Beverley. I realised that I’d never been this close to her before; her skin was very pale and perfect in the same way that Beverley’s was. She refused a beer but tentatively accepted a piece of pizza. When she ate she turned her face away and held her hand so that it obscured her mouth.

‘When are you going to sort out Father Thames?’ asked Beverley. ‘Mum’s getting impatient and the Richmond posse is getting restless.’

‘Richmond posse,’ said Lesley, and snorted.

‘We’ve got to find him first,’ I said.

‘How hard can it be?’ said Beverley. ‘He’s got to be close to the river. Hire a boat, go upstream and stop when you get there.’

‘How would we know when we got there?’

‘I’d know.’

‘Then why don’t you come with us, then?’

‘No way,’ said Beverley. ‘You’re not getting me up past Teddington Lock. I’m strictly tidal, I am.’

Suddenly Molly’s head whipped round to face the door, and a moment later somebody knocked. Beverley looked at me but I shrugged – I wasn’t expecting anyone. I hit mute on the remote and got up to answer. It was Inspector Nightingale dressed in the blue polo shirt and blazer which I recognised as being the closest thing he ever got to casual dress. I stared at him stupidly for a moment, and then invited him in.

‘I just wanted to see what you’d done with the place,’ he said.

Molly shot to her feet as soon as Nightingale came into the room, Lesley got up because he was a senior officer and Beverley stood either from some vestigial politeness or in anticipation of a quick getaway. I introduced Beverley, who he’d met only briefly when she was ten.

‘Would you like a beer, sir?’ I asked.

‘Thank you,’ he said. ‘Call me Thomas, please.’

Which was just not going to happen. I handed him a bottle and indicated the chaise longue. He sat carefully and upright at one end. I sat at the other end while Beverley flopped into the middle of the sofa, Lesley sat slightly to attention and poor Molly bobbed a couple of times before perching right on the edge. She kept her eyes resolutely downcast.

‘That’s a very large television,’ said Nightingale.

‘It’s a plasma TV,’ I said. Nightingale nodded sagely while out of his sight Beverley rolled her eyes.

‘Is there something wrong with the sound?’ he asked.

‘No,’ I said. ‘I have it on mute.’ I found the remote and we got ten seconds of Beat the Rest before I got the volume under control.

‘That’s very clear,’ said Nightingale. ‘It’s like having your own cinema.’

We sat in silence for a moment, everyone, no doubt, appreciating the theatre-quality surround sound.

I offered Nightingale a slice of pizza, but he explained that he’d already eaten. He asked after Beverley’s mother, and was told she was fine. He finished his beer and stood up.

‘I really must be on my way,’ he said. ‘Thank you for the beer.’

We all stood up and I walked him to the door. When he left I heard Lesley sigh and flop back on the sofa. I almost shouted when Molly suddenly slid past me in a rustle of fabric and slipped out the door.

‘Awkward,’ said Beverley.

‘You don’t think she and Nightingale … ?’ asked Lesley.

‘Ew,’ said Beverley. ‘That’s just wrong.’

‘I thought you and her were friends?’ I asked.

‘Yeah, but she’s like a creature of the night,’ said Beverley. ‘And he’s old.’

‘He’s not that old,’ said Lesley.

‘Yes he is,’ said Beverley, but however many hints I dropped that evening, she wouldn’t say any more.