The London Underground CCTV hub still had the smell of new carpets and fresh paint. Twenty wall-mounted monitors faced the row of desks, behind which three operators switched deftly between cameras using joysticks and computer keyboards. In one corner a door led to the editing suite, where footage could be captured, enhanced and circulated to investigating officers. Kelly signed in and made her way to Craig’s station on the far side, one eye on a follow at Kings’ Cross being monitored by one of the other operators.

‘He’s passing Boots now … something dumped in the bin below the clock. Green hoody, black Adidas tracksuit bottoms, white trainers.’

A uniformed officer ran across the screen, gaining on the tracksuited figure, who was now level with Claire’s Accessories. All around them stood people with suitcases, briefcases, shopping bags. They looked up at the huge screens above their heads; waiting for platform information, train times, delays. Oblivious to the crimes that went on around them every day.

‘Hi, Kelly, how’s Met life treating you?’

Kelly liked Craig. He was in his early twenties and desperate to join the job. He soaked up everything officers said, and had better instincts than half the coppers Kelly had worked with, but the fitness test was proving a bit of a challenge.

‘It’s great, I’m loving it. How’s the training going?’

Craig looked proud. He patted his not-inconsiderable stomach. ‘Four pounds down this week. Slimming World.’

‘Good for you. Can you help me find someone?’

Locating Luke Friedland on CCTV was easy; Zoe Walker’s timings were spot on. The platform at Whitechapel was too crowded for Kelly to clearly see Zoe, but after the train had pulled away, taking the crowd of people with it, the camera footage showed her standing opposite a tall man.

Luke Friedland.

Assuming that really was his name.

If she hadn’t been aware of the context, Kelly would have taken them for a couple. They seemed at ease together, Friedland touching Zoe lightly on the arm as they said goodbye.

‘Play that clip again for me,’ she asked Craig.

A swell in the crowd, like a muted Mexican wave, indicated some sort of commotion as the train approached, but it was swiftly replaced by the surge of commuters getting on to the train. The camera was too far away to see exactly what had happened to make Zoe trip.

Kelly’s phone vibrated against the desk. She looked down to see a text message from Lexi, and flipped the phone on to its front so she could ignore it. Let Lexi leave another voicemail – Kelly didn’t want to speak to her.

U dont undrstnd, Lexi’s last text had said.

Kelly didn’t. What was the point in the job she and her colleagues did? In the CPS files, the court system, the prison service? What was the point in fighting for justice if victims – people like Lexi – couldn’t be bothered to support proceedings?

She gave Craig the second date and time. Tuesday 24 November; around 1830 hours. Zoe’s second encounter with Friedland; when he had accompanied Zoe from the train at Crystal Palace to the exit, then asked her out for a drink. Had he downloaded other women’s profiles from the website? Tried the same approach with them? Andrew Robinson had seemed confident his Cyber Crime team would identify the man behind the website, but how long would it take? In the meantime Kelly was treating the case in the same way she’d tackle a drugs ring; from the bottom up. Gordon Tillman had refused to answer her questions, but perhaps Luke Friedland would be more talkative.

‘This him?’ Craig pressed pause and Kelly nodded.

They were walking towards the barriers; Kelly recognised Zoe’s red waterproof jacket, and the more formal overcoat she’d seen Friedland wearing in the previous clip. Exactly as Zoe had said in her statement, as they approached the ticket barriers, Friedland waited, letting Zoe go first.

Kelly smiled as she saw Friedland tap his Oyster on the barrier. ‘Gotcha,’ she muttered, noting down the precise time on the screen. Picking up the phone, she dialled from memory. ‘Hey, Brian, what’s new?’

‘Same shit, different day; you know how it is,’ Brian said cheerfully. ‘How’s the secondment going?’

‘Loving it.’

‘What can I do you for?’

‘Tuesday twenty-fourth November, Crystal Palace, second barrier from the left, 1837. If it helps, the system should show a Mrs Zoe Walker immediately preceding it.’

‘Give me a tick.’

Kelly heard the tapping of Brian’s keyboard. He was singing under his breath, and Kelly recognised the same tuneless refrain he’d been humming ever since she had known him. Brian had done his thirty years in the job, kick-started his pension then returned the next day to a new job with London Underground.

‘I’d be bored at home,’ he’d told Kelly, when she’d questioned why he wasn’t off enjoying his retirement. After thirty years working in London there was nothing Brian didn’t know about the city; when he finally retired he’d be hard to replace.

‘Any idea who you’re after, Kelly?’

‘Definitely a man,’ she said, ‘possibly a Luke Friedland.’

Another pause, then Brian chuckled; a throaty, phlegmy sound fuelled by coffee and Benson & Hedges. ‘Not very imaginative, your chap. His Oyster’s registered to a Luke Harris. Want to have a stab at what street he lives on?’

‘Friedland Street?’

‘Got it in one.’

They were waiting for him when he got home from work, stepping out of the car as he paused to enter his door code.

‘Could we have a quick word?’ Kelly said, showing her warrant card and watching Harris intently. Was she imagining it, or was there a flash of panic in his eyes?

‘What about?’

‘Shall we go up?’

‘It’s not terribly convenient; I’ve got a lot of work to do tonight. Perhaps you could leave a number …’

‘We can take you down the station, if you’d prefer?’ Nick said, moving from behind Kelly to stand next to her. Harris looked from one to the other.

‘You’d better come in.’

Luke Harris lived in a penthouse apartment in W1, the highest of six floors housing more modest flats. They stepped out of the lift into a vast open-plan space, the gleaming white surfaces of a rarely used kitchen to their left.

‘Very nice,’ Nick said, walking across the living room and looking out at the city. To the right the BT Tower loomed over its neighbours, and Kelly could see the Shard and Heron Tower in the distance. In the centre of the room two overstuffed sofas sat facing each other, separated by a huge glass coffee table; its surfaces piled with glossy travel books. ‘Read all these, have you?’

Harris was nervous, tugging at his tie and looking first at Kelly, then at Nick. ‘What’s all this about?’

‘Does the name Zoe Walker mean anything to you?’

‘I’m afraid not.’

‘You asked her out for a drink last week, outside Crystal Palace station.’

‘Ah! Yes, of course. Zoe. She said no.’ Kelly detected a note of indignation which didn’t match the careless shrug Harris had given.

‘Unusual for a woman to resist your charms?’ Kelly said, her voice thick with sarcasm. Harris had the grace to blush slightly.

‘Not at all. It’s only that we’d got on rather well, I thought, in the short time we’d spent together. And although she was attractive she must have been pushing forty, so …’ he tailed off under Kelly’s withering stare.

‘And you thought she might be a bit more grateful?’

Harris said nothing.

‘How did you meet Zoe Walker?’ Nick turned away from the floor-to-ceiling windows and walked to the middle of the room. Harris hadn’t invited her to take a seat, and had remained standing himself, so Kelly had done the same. The DI had no such reservations. He sat heavily on one of the sofas, the cushion billowing out either side of him. Kelly followed his lead. Reluctantly, as though he had up to that point hoped they wouldn’t be staying long, Harris sat down opposite them.

‘We got chatting on the Underground on Monday. Then we bumped into each other again and seemed to hit it off.’ He shrugged again, but there was something forced about it. ‘It’s not a crime to ask someone out, is it?’

‘You met on the Tube?’ Kelly said.

‘Yes.’

‘Completely by chance?’

Harris paused. ‘Yes. Look, this is all quite absurd. I’ve got work to do, so if you don’t mind—’ He began to stand up.

‘You didn’t buy her commute details on a website called find the one?’ Kelly kept her tone casual, enjoying the look on Harris’s face, which oscillated between shock and fear. He sat down, and Kelly waited for him to speak.

The pause seemed to go on for ever.

‘Are you arresting me?’

‘Should I be?’

Kelly let the silence answer for her. Had he committed an offence? It wasn’t a crime to ask Zoe Walker out for a drink, but if he’d been following her …

Gordon Tillman had been charged with rape, remanded and put before a Saturday morning magistrates court. On his solicitor’s advice Tillman had gone no comment to all the questions put to him, despite Kelly’s suggestion that he was only making the situation worse.

‘Who’s behind the website, Gordon?’ Kelly had asked again. ‘The courts will look far more favourably on you if you help us out.’

Tillman had looked at his solicitor, who was quick to answer on his behalf.

‘That’s a bold promise, PC Swift, and one you are not at liberty to make. I have advised my client to make no further comment.’

There had been a half-hearted attempt at a bail application at court, based on Tillman’s previous good character, his standing in the community, and the impact his absence would have on his career; but the speed with which the magistrate refused the request suggested he had made up his mind some time earlier.

They hadn’t managed to get any information out of Tillman, but perhaps Luke Harris would prove more forthcoming. The stakes were lower; no allegation of rape, no custody-issue tracksuit, no time in a cell. Softly, softly.

‘The website,’ Kelly prompted now.

Luke leaned his elbows on his knees and rested his head between splayed-out fingers. ‘I joined a few weeks ago,’ he muttered to the thick pile of the rug beneath the coffee table. ‘Someone at work put me on to it. Zoe’s was the first profile I’d downloaded.’

Highly unlikely, Kelly thought, but she decided to let it go. For now. ‘So why not tell us that when we first asked?’

Harris looked up. ‘It’s run on the QT, as I understand it. Members are encouraged to be discreet.’

‘By whom?’ Nick said. ‘Who runs the site, Luke?’

‘I don’t know.’ He looked up. ‘I don’t! That’s like asking me who owns Wikipedia, or Google Earth. It’s just a site I use – I’ve got no idea who runs it.’

‘How did you find out about it?’

‘I told you, someone at work.’

‘Who?’

‘I don’t remember.’ Luke became more agitated with each question Nick fired at him.

‘Try.’

He rubbed his forehead. ‘A load of us were talking in the pub after work. It was a bit hardcore. Some of the guys had been to a strip club at the weekend – there was a lot of banter about it. You know what it’s like when lads get together.’ This was directed at Nick, who remained expressionless. ‘Someone mentioned the website. They said I’d need a password to open an account – that it was hidden in the phone number on an advert in the back of the London Gazette. A sort of secret code, just for people in the know. I wasn’t going to look but I was curious and …’ He tailed off, looking between Nick and Kelly. ‘I wasn’t doing anything wrong.’

‘I think you should leave us to decide that,’ Nick said. ‘So you downloaded Zoe Walker’s details, then you followed her.’

‘I didn’t follow her! I’m not a stalker. I just engineered it so I’d bump into her, nothing else. Look, all this’ – he waved an arm around, encompassing the penthouse – ‘is great, but I work bloody hard for it. I’m in the office seven days a week, on conference calls to the States every night … it doesn’t leave much time to meet women. The website gives me a leg-up, that’s all.’

A leg-over, Kelly thought, catching Nick’s eye. ‘Tell me what happened on the platform at Whitechapel, the first time you spoke to Zoe Walker.’

That shifty look again from Harris; his eyes flicking up to the left.

‘What do you mean?’

‘We’ve got a statement from Zoe,’ Kelly said, chancing her arm. ‘She’s told us everything.’

Harris closed his eyes briefly. When he opened them, he avoided eye contact, staring instead at an illustrated guide to Italy in front of him on the coffee table. ‘I’d tried to get chatting to her that morning. I found her on the Overground, right where her profile said she’d be. I tried to speak to her, but she ignored me. I decided if I helped her with something it would break the ice: I thought I could give up my seat for her, or carry her shopping or something. But nothing like that came up. Then I was behind her at Whitechapel, and she was standing really close to the edge of the platform, and …’ He stopped talking, his eyes still fixed on the book in front of him.

‘Go on.’

‘I pushed her.’

Kelly took an involuntary breath. Next to her she felt Nick sit up. So much for the softly, softly approach.

‘I pulled her to safety instantly. She was never in any danger. Women like being rescued, don’t they?’

Kelly bit back her instinctive response. She glanced at Nick, who nodded. Kelly stood up. ‘Luke Harris, I’m arresting you on suspicion of the attempted murder of Zoe Walker. You do not have to say anything, but it may harm your defence if you do not mention, when questioned, something you later rely on in court.’