‘What sort of game?’ I say. Melissa smiles. She is still sitting at her desk, spinning the chair so she can face us. She looks at the computer screen.

‘More than a hundred hits already.’ She looks at Katie. ‘You’re a popular girl.’

My stomach lurches. ‘You’re not putting her on that website.’

‘She’s already on there.’ Melissa clicks again, and I see Katie’s photo on the screen, pouting at us with a careless confidence in stark contrast to our current situation. Katie cries out and I put my arm around her, pulling her towards me so fiercely her chair scrapes across the floor.

‘So here’s how it will work.’ Melissa is using her business voice; the one she adopts when she’s on the phone to suppliers, or cajoling the bank manager into yet another loan. I’ve never heard her use it with me before, and it makes my blood run cold. ‘I’ve made Katie’s profile free to download for a limited period, and I’ve sent the link to all members.’

There’s another ping from the computer; a notification box appears, then another and another.

‘As you can tell, they’re quick off the mark. Hardly surprising, when you think they’re usually paying up to five hundred quid for someone far less …’ she takes time choosing the right word, finally settling on one that makes me sick to my stomach, ‘enticing.’

‘She’s not going anywhere.’

‘Oh, come on, now. Where’s your sense of adventure? Not all my clients have nefarious aims, you know. Some of them are really rather romantic.’

‘She’s not going.’

‘Then I’m afraid it’s going to end very badly for both of you.’

‘What do you mean?’

She ignores my question. ‘Here are the rules. Katie follows her normal commute, and if she gets to the restaurant without any … shall we say interruptions … then you win and I let you go. If she doesn’t … well, you both lose.’

‘That’s sick,’ Katie says.

Melissa looks at her, a sneer on her face. ‘Oh, come on, Katie, it’s not like you to pass up the opportunity to be in the limelight.’

‘What’s that supposed to mean?’

‘This is your chance to be star of the show. We all know you’re not happy unless you’re the centre of attention. Never mind that Justin might have wanted a chance, or one of your friends. It always has to be about you, doesn’t it? Like mother, like daughter.’

I’m stunned by the hatred in her voice. Katie is crying, as shocked as I am.

‘So,’ Melissa says, ‘that’s the game. Ready to play? Or would you rather skip straight to the part where you both lose?’ She tests the blade of the knife on her thumbnail, where it’s too sharp to slide smoothly across the red lacquer Melissa always wears.

‘You’re not using my daughter as bait for a bunch of sick men. I’d rather die.’

Melissa shrugs. ‘Your call.’ She stands up and walks towards me, the knife held out in front of her.

‘No!’ Katie screams. She clings to me, tears streaming down her face. ‘I’ll do it, I’ll go – I won’t let her hurt you.’

‘Katie, I’m not letting you do it. You’ll get hurt.’

‘And if I don’t, we both will! Don’t you understand? She’s mad!’

I glance at Melissa but she seems entirely unperturbed by Katie’s accusation. There’s no sign of agitation, or of anger, which makes her actions even more terrifying. She would push that knife into me, I realise, and not even break into a sweat. I’m struggling to accept that the woman I thought was my friend – the woman I thought I knew – is someone else entirely. Someone who hates me. Who resents me so deeply for being a mother that she’s prepared to hurt me; to hurt my daughter.

Katie squeezes my shoulder. ‘I can do this, Mum. The Tube will be busy – there’ll be people everywhere – no one’s going to hurt me.’

‘But, Katie, they have been hurt! Women have been murdered. Raped! You can’t go.’ Even as I say it, I’m thinking of the alternative. If Katie stays here, what will happen to her? I’m in no doubt now that Melissa is going to kill me, but I won’t let her kill Katie too.

‘The other women didn’t know they were being watched. I do. I’ll have the advantage. And I know that route, Mum. I’ll know if someone’s following me.’

‘No, Katie.’

‘I can do this. I want to do it.’ She’s not crying any more; her face is set with a determination I know so well I catch my breath. She thinks she’s saving me. She really thinks she can play this game – that she can cross London without being caught – and that winning the game means Melissa will spare me.

She’s wrong – Melissa won’t let me go – but in trying to save me, I can save Katie. Out there, she has a fighting chance. In here, we’re already dead.

‘Okay,’ I tell her. It feels like a betrayal.

She stands up and looks at Melissa. Her chin juts out defiantly, and for a second I’m reminded of her character in the play, hiding her identity behind boy’s clothing and clever words. If Katie’s scared, she isn’t showing it.

‘What do I have to do?’

‘You just have to go to work. Nothing simpler. You’ll leave in,’ she checks the computer screen, ‘five minutes, and you’ll follow your usual route to the restaurant. You’ll give me your phone, you won’t stop, or change your routine, and you won’t do anything stupid like call for help or try to contact the police.’

Katie hands over her mobile. Melissa walks to her desk and presses a series of keys. The computer screen switches to a colour CCTV image I recognise; it’s looking out of Crystal Palace Tube station. I can see the taxi rank to the left, and the graffiti on the wall that’s been there for as long as I can remember. As we watch, a woman hurries into the station, checking her watch.

‘Step out of line,’ Melissa continues, ‘and I’ll know. And it doesn’t take a genius to work out what will happen to your mother.’

Katie bites her lip.

‘You don’t have to do this,’ I say softly.

She tosses her hair. ‘It’s fine. I’m not going to let anything happen to me, Mum. Or you.’ She has a look of grim determination in her eyes, but I know her too well to believe she feels as confident as she looks. She’s playing a part, but this isn’t a play. It isn’t a game, whatever Melissa calls it. Whatever happens, someone’s going to get hurt.

‘Time to go,’ Melissa says.

I hug Katie so tightly it forces the air from my lungs. ‘Be careful.’ I must have said the same thing thousands of times since becoming a mother, each time a shortcut for something more.

‘Be careful,’ when she was ten months old and cruising the furniture. Don’t break anything, I really meant, Watch that vase.

‘Be careful,’ when she learned to ride her bike. Watch out for cars, I could have said.

‘Be careful,’ the first time she got serious about a boyfriend. Don’t get hurt, I meant. Don’t get pregnant.

‘Be careful,’ I say now. Don’t let them catch you. Keep your eyes open. Be quicker than them. Run fast.

‘I will be. I love you, Mum.’

Pretend it’s a normal day, I tell myself, as tears well in my eyes. Pretend she’s just going to work, and that she’ll be home later and we’ll put Desperate Housewives on Netflix and eat pizza. Pretend this isn’t the last time you’ll ever see her. I’m crying openly now, and so is Katie, her temporary bravado too fragile to survive such an onslaught of emotion. I want to tell her to look out for Justin when I’m gone, to make sure Matt doesn’t let him go off the rails, but doing so would acknowledge what I don’t want in her head: that I won’t be here when she gets back. If she gets back.

‘I love you too.’

I take in every last detail of her: the way her hair smells; the smudge of lip gloss in the crease of her mouth. I fix her so firmly in my mind that whatever happens in the next hour I know it will be her face I see in my head when I die.

My baby girl.

‘Enough, now.’ Melissa opens the kitchen door and Katie walks along the narrow hall towards the front of the house. This is my chance, I think. I consider charging after Katie as the front door opens, pushing us both outside and running; running to safety. But although the knife hangs by Melissa’s side, she is gripping it so tightly her knuckles have whitened. She would use it in a heartbeat.

Knives.

I should have thought of it instantly. The knife block, now missing one resident, still contains a carving knife and three vegetable knives, in descending sizes. I hear the sound of a key in the lock and then, all too quickly, the door slams again and I’m assaulted by an image of Katie, walking towards the Tube station. Walking towards danger. Run away, I beg her silently. Go the opposite way. Find a phone box. Tell the police.

I know she won’t. She thinks Melissa will kill me if she doesn’t appear on that CCTV camera in precisely eight minutes.

I know she’ll kill me even if she does.

When Melissa returns I’m halfway between the table and the kitchen counter. She’s carrying something she must have picked up in the hall. A roll of duct tape.

‘Where are you going? Get over there.’ She gestures with the tip of the knife, and I need no further persuasion. Melissa moves my chair so it is facing her computer. I sit.

‘Put your hands behind your back.’

I comply, and hear the distinctive ripping sound of duct tape, torn off into strips. Melissa wraps a strip around my wrists, then pushes the tape around the wooden struts of the chair, so I can’t move my arms. She tears off two more strips and secures my ankles to the legs of the chair.

I watch the clock in the right-hand corner of the screen.

Six minutes to go.

I’m comforted by the thought that Katie’s journey to work is on busy routes, and that it’s still light. There are no dark alleyways in which she could be trapped, and so surely if she keeps her wits about her, she will be okay. The women who have become victims – Tania Beckett, Laura Keen, Cathy Tanning – they didn’t know they were being targeted. Katie knows. Katie has the upper hand.

‘Ready for the show?’ Melissa says.

‘I’m not watching.’ But I find I can’t help myself. I have a sudden memory of taking Katie to hospital when she was a baby, and forcing myself to look while they put a cannula into her tiny hand, rehydrating her after a bad sickness bug. I wanted so badly to take it all away from her, but if I couldn’t do that, then the least I could do was carry the pain of seeing her suffer; live through it with her.

The score across the front of my neck has already started to scab over, and the tightness pulls at my skin and makes it itch. I stretch my neck in an attempt to relieve the feeling, releasing fresh blood that drips on to my lap.

Four minutes.

We watch the screen in silence. There’s so much I want to know, but I don’t want to hear Melissa’s voice. I indulge myself with a fantasy in which even now the police are speeding towards Anerley Road. Any moment now I’ll hear a crash as police officers break open the front door. It’s so real I strain to hear the police sirens. There is nothing there.

Two minutes.

It seems an eternity until we see Katie appear on the CCTV image. She doesn’t pause, but she looks up at the camera as she approaches, staring straight at us until she passes underneath the camera and out of view. I see you, I mouth. I’m with you. I can’t stop my tears falling.

‘We can’t follow her through the ticket barrier, unfortunately,’ Melissa’s tone is companionable – chatty, almost – as though we’re working on a project together. It’s unnerving, more so than if she were shouting at me, or threatening me. ‘But we’ll pick her up again once she’s on the platform.’

She moves her mouse across the screen and I catch sight of a list of what I assume must be cameras: Aldgate East – entrance; Angel – entrance; Angel – southbound platform; Angel – northbound platform; Bakerloo – ticket barriers … The list goes on and on.

‘Quite a few of the early profiles aren’t in the right area for the cameras I can access,’ Melissa explains, ‘but we’ll be able to get most of Katie’s commute. Look – there she is.’

Katie is standing on the platform, her hands thrust into her pockets. She’s looking around and I hope she’s searching for cameras, or working out which of her fellow passengers might be a threat. I see a man in a suit and overcoat approach her. Katie steps back slightly, and I dig my nails into my closed palms until he passes without checking his pace. My heart is pounding.

‘Quite the little actress, isn’t she?’

I ignore her. The Overground train arrives and Katie steps on, the doors closing and swallowing her up all too quickly. I want Melissa to click on to the next camera, but she doesn’t move. She picks a piece of cotton from her jacket and frowns at it, before letting it float from her fingers on to the floor. My fantasy evolves: I imagine Simon returning from his interview; finding the house empty – the door unlocked – and somehow knowing I am next door. Rescuing me. My imaginings grow in detail and absurdity in inverse proportion to my dwindling hope.

No one is coming.

I will die here, in Melissa’s house. Will she dispose of my body, I wonder, or leave me here, festering, for Neil to find when he returns from his work trip?

‘Where will you go?’ I ask her. She turns to look at me. ‘Once you’ve killed me. Where will you go?’ She starts to say something – to deny that I’m going to die – then she stops. There’s a flash of what looks like respect in her eyes, and then it’s gone. She shrugs.

‘Costa Rica. Japan. The Philippines. There are still plenty of countries without extradition agreements.’

I wonder how long it will take them to find me. Whether Melissa could make it to another country by then. ‘They’ll stop you at passport control,’ I say, more confidently than I feel.

She looks at me scornfully. ‘Only if I use my own passport.’

‘How—’ I can’t find the words. I have stumbled into a parallel universe, in which people wield knives and use fake passports and murder their friends. I suddenly realise something. Melissa is clever, but she’s not that clever. ‘How did you learn all this?’

‘All what?’ She’s distracted, tapping the keyboard. Bored with the conversation.

‘The CCTV, a false passport. PC Swift said the adverts were placed by a man; that he had a mailbox set up in his name. The website is untraceable. You had help – you must have done.’

‘That’s rather insulting, Zoe. I think you underestimate me.’ She doesn’t look at me, and I know she’s lying. She couldn’t have done this alone. Is Neil really away with work? Or is he upstairs? Listening. Waiting until reinforcements are needed. I glance nervously towards the ceiling. Did I imagine the creak in the floorboards?

‘That’s been fifteen minutes,’ Melissa says abruptly, looking at her watch. ‘I can’t get into the Overground trains, but the next camera will get her changing at Canada Water.’ She clicks on the next camera and I see another platform; a group of schoolchildren being shepherded away from the edge by three teachers wearing high-visibility tabards. A train arrives and I scour the screen for Katie, but can’t find her. My heart beats faster: has something happened to her already? On that short journey from Crystal Palace to Canada Water? But then I catch a glimpse of a white Puffa coat, and there she is, her hands still pushed into her pockets, her head still turning this way and that, looking at everyone she passes. I let out the breath I’ve been holding.

Katie goes out of sight, and despite Melissa bringing up two more cameras, we don’t see her again until she’s waiting on the Jubilee line platform. She’s standing close to the platform edge and I want to tell her to step away, that someone could push her in front of the train. Watching her like this, on CCTV, is like watching a film in which you know something terrible is about to happen to the main character, and you scream at them not to be so stupid.

Don’t go outside, don’t ignore that sound you heard … haven’t you read the script? Don’t you know what happens next?

I remind myself that Katie has read the script. She knows what the danger is, she just doesn’t know exactly where it’s coming from.

There’s a man standing behind Katie, and to her left. He’s watching her. I can’t see his face – the camera is too far away – but his head is turned towards her and it moves slightly as he looks her up and down. He takes a step closer and I grip the edge of my seat, leaning forward in a vain attempt to see more. There are other people on the platform – why aren’t they looking the right way? They won’t see if he does something. I used to feel so safe on the Underground. So many cameras, so many people all around. But no one’s watching, not really. Everyone’s travelling in their own little bubble, oblivious to what’s happening to their fellow commuters.

I say her name under my breath and as if she’s heard me she turns around. Looks at the man. He steps closer and immediately Katie backs off. I can’t read her body language – is she frightened? She walks to the other end of the platform. Melissa shifts in her chair and I look at her. She’s gazing intently at the screen, but she isn’t sitting forward, tense in her chair, like I am. She’s leaning backwards, her elbows resting on the arms of her chair, and her fingertips pressed together. A small smile plays across her lips.

‘Fascinating,’ she says. ‘I always liked the idea that the women didn’t know they were being followed, but this adds something quite interesting. Cat and mouse on the Underground. It might work rather well as an extra package for members.’ Her flippancy revolts me.

The man on the platform hasn’t followed Katie to the other end of the platform, but as the train arrives, and a surge of tourists and commuters disembark, I see him move through the mêlée towards her. He doesn’t get on at the same place as her, and I’m feeling relieved when I realise he has nevertheless chosen the same carriage.

‘Can you get into the camera on that train? I want to see it. I want to see what’s happening on the train!’

‘Addictive, isn’t it? No, I’ve tried but it’s very secure. We’ve got’ – she checks another tab, open on the computer – ‘seven minutes till she gets to Waterloo.’ She drums her fingers on the desk.

‘The carriage is busy. No one will try anything on a busy train.’ I say it to myself as much as to Melissa.

If Katie cried out, would someone do something? I’ve always taught her to make a noise if something happens. ‘Be loud about it,’ I told her. ‘If some perv pushes himself against you, don’t tell him, tell everyone. Shout, “Stop touching me this instant!” Let the whole carriage know. They might not do anything, but he’ll stop straight away, you’ll see.’

It’s only four minutes from Waterloo to Leicester Square. I know because Melissa has told me, and because every second feels like an hour. As soon as we lose Katie into a Northern line train at Waterloo, Melissa brings a new image on to the screen; the camera looking towards the bottom of the escalators leading up to Leicester Square.

We watch in silence until she appears.

‘There she is.’ Melissa points to Katie. Instantly I look for the man I saw approaching her on the platform, and when I find him a couple of yards behind her my chest tightens.

‘That man …’ I say, but I trail off because – what is there to say?

‘He’s persistent, isn’t he?’

‘Do you know who he is? Where he comes from? How old he is?’ I don’t know why any of these things matter.

‘The profile’s been downloaded almost two hundred times,’ Melissa says. ‘It could be any one of them.’

The man pushes past a woman with a buggy. Katie steps on to the escalator.

Keep walking, I say in my head, but she stands still, and the man walks up on the left-hand side and then slots in on the right to stand behind her. He puts a hand on her arm and leans in. He’s saying something to her. Katie shakes her head, and then they reach the top of the escalator and out of view.

‘The next camera! Get the next camera!’

Melissa responds with deliberate slowness, enjoying my panic. There are lots of people at Leicester Square, and when she finally pulls up another CCTV image I can’t immediately see Katie. But then I spot her, walking alongside the man from the train. My heart races: something isn’t right. Katie is walking at an odd angle, bent to one side. Her head is bowed and although she doesn’t look as though she’s fighting him, everything about her body language tells me she can’t get away. I look closer and realise he is gripping the top of her left arm with his right hand. With his other hand he is gripping her wrist: it is the pressure on this arm pulling her off balance. He must have a weapon. He must be threatening her. Otherwise why isn’t she screaming? Running? Fighting?

I watch Katie walk towards the ticket barriers with this man, her arm pulled awkwardly across the front of his chest. There are two ticket collectors standing by a Tube map, chatting, and I will them to notice something is wrong, but they pay no attention. How can this be happening in broad daylight? Why is no one seeing what I’m seeing?

I can’t take my eyes off the screen.

Surely once Katie and the man reach the barriers he’ll have to let her go? That will be her chance to get away. I know Katie, she’ll be planning it now – working out where to run, which exit to take. I feel a surge of adrenaline. She’ll do it – she’ll get away from him.

But they don’t reach the gates. Instead the man leads her to the left of the concourse, where there is an empty information kiosk and a door marked ‘no entry’. He glances behind, as if to see whether they’re being observed.

And then my blood runs cold as I see him open the door and take Katie inside.