Melissa stands in the doorway between the hall and the kitchen. She registers the horrified look on Katie’s face; the unfolded Underground map in my hand, and slowly the smile disappears from her face. I realise I’m hoping she’ll deny it; that she’ll produce some plausible explanation for the evidence I’m holding.

She doesn’t even try. Instead she gives a deep sigh, as though our actions are tedious in the extreme.

‘It’s very bad manners to rifle through someone’s personal belongings,’ she says, and I have to swallow the automatic apology it prompts. She walks across the kitchen, her heels clicking against the tiled floor, and takes the Underground map from my hand. I realise I’m holding my breath, but when I let it out there isn’t anything there; my chest feels tight, as though someone is pushing against it. I watch her refold the map, tutting when a crease bends the wrong way, but not hurried, not panicked in the slightest. Her coolness disorientates me, and I have to remind myself that the evidence is incontrovertible. Melissa is behind the website; behind the London Gazette adverts. It’s Melissa who has been hunting women across London; selling their commutes so that men can hunt them too.

‘Why?’ I ask her. She doesn’t answer.

‘You’d better sit down,’ she says instead, gesturing to the long white table.

‘No.’

Melissa gives an exasperated sigh. ‘Zoe, don’t make this any more difficult than it’s going to be. Sit down.’

‘You can’t keep us here.’

She laughs, then; a humourless bark that says she can do exactly what she wants. She walks the few steps towards the kitchen counter; an expanse of black granite broken only by the coffee machine and a knife block next to the hob. Her hand hovers over the block for a second, her index finger playing a silent game of Eenie, Meenie, Miney, Mo, before she pulls out a black-handled knife around six inches long.

‘Can’t I?’ she says.

I sink slowly into the chair nearest to me. I tug at Katie’s arm and after a moment she does the same.

‘You won’t get away with this, Melissa,’ I tell her. ‘The police will be here any minute now.’

‘I very much doubt that. Judging from the updates you’ve so helpfully shared with me over the last few weeks, the police have proved themselves to be largely incompetent.’

‘But you told PC Swift where we were. She’ll—’ I stop even before I see the pitying look on Melissa’s face. How stupid of me. Of course Melissa didn’t really call Kelly Swift. The realisation is like a punch to the stomach and I fold forward in my chair, suddenly spent. There are no police coming. My panic alarm is in my bag at home. No one knows we’re here.

‘You’re sick,’ Katie spits, ‘or mad. Or both.’ There’s more than just anger in her voice. I think of all the time Katie has spent in this kitchen over the years; baking cakes, doing her homework, talking to Melissa in a way that sometimes isn’t possible between a mother and daughter, no matter how close they are. I try to imagine how she must feel, then I realise I’m already there. Lied to. Taken advantage of. Betrayed.

‘Neither. I saw a business opportunity and I took it.’ Melissa walks towards us, the knife held casually in one hand as though she has been interrupted in the middle of preparing dinner.

‘This isn’t a business!’ I say, so outraged I stumble over the words.

‘It most certainly is a business, and a very successful one. I had fifty clients within a fortnight of setting up the website, with more joining every day.’ She sounds like an advert for a franchise opportunity; like she’s bragging about adding to her chain of coffee shops.

She sits opposite us. ‘They’re so stupid. Commuters. You see them, every day, oblivious to the world around them. Plugged into their iPods, staring at their phones, reading their papers. Taking the same route every day, sitting in the same seat, standing on the same spot on the platform.’

‘They’re just going to work,’ I say.

‘You see the same ones every day. I was watching this woman once, doing her make-up on the Central line. I’d seen her a few times, and she always had the same routine. She’d wait till Holland Park, then she’d get out her make-up bag and start plastering her face. Powder first, then eye-shadow, mascara, lipstick. As the train slowed down at Marble Arch she’d be putting her make-up bag away. I watched her this one time, and as I looked away I caught a man watching her too, with a look in his eyes that suggested he was thinking about more than her face. That’s when I first had the idea.’

‘Why me?’ And as I say it I can’t believe it has only now occurred to me to ask. ‘Why put me on the website?’

‘I needed a few older women.’ She shrugs. ‘There’s no accounting for people’s tastes.’

‘But I’m your friend!’ Even as I say it I hate myself for how pathetic it sounds, like a schoolyard catfight over who plays with whom.

Melissa’s lips tighten. She stands up abruptly, striding towards the bi-fold doors and gazing into the garden. It’s several seconds before she speaks.

‘I’ve never known anyone moan about their life as much as you do.’ I’d been expecting something different; some indiscretion, committed years ago. Not this. ‘I had my kids too young,’ she mimics.

‘I’ve never said that.’ I look at Katie. ‘I’ve never regretted having you. Either of you.’

‘You walk out on a textbook husband – solvent, funny, hands-on with the kids – and replace him with someone equally textbook.’

‘You have no idea what my marriage to Matt was like. Or what my relationship with Simon is like, come to that.’ At the thought of Simon, guilt overwhelms me. How could I have thought he was responsible for the website? I think of the names, and the scribbled threats I found in Simon’s desk drawer, and for a second I doubt myself, then I realise what they are: research notes. He’s used the Moleskine for exactly the purpose it was intended; to plan his novel. Relief makes me smile, and Melissa looks at me with venom in her eyes.

‘It’s all so easy for you, isn’t it, Zoe? Yet you never stop complaining.’

‘Easy?’ I’d laugh, were it not for the knife in her hand, which catches the light from the skylight and throws rainbows around the room.

‘—and from the second you move in next door it’s the poor me routine. Single mum, struggling to pay the bills, bursting into tears every five minutes.’

‘It was a difficult time,’ I say, in my defence, speaking more to Katie than to Melissa. Katie reaches for my hand; gives me the silent support I need.

‘Whatever you asked for, I gave you. Money, a job, help looking after the kids.’ She spins round; I hear her heels scrape on the tiles then she bends over me, her hair falling over mine, and hisses in my ear. ‘What have you ever given me?’

‘I—’ My mind is blank. Surely I must have done something? But there’s nothing. Melissa and Neil have no children, no pets to look after, no houseplants to water when they’re away on holiday. There’s more to friendship than that though, isn’t there? Do the scales of friendship have to balance so absolutely? ‘You’re jealous,’ I say, and it seems such an insignificant word to justify something of this magnitude; of this horror.

Melissa looks at me as though she’s stepped in something unpleasant. ‘Jealous? Of you?’

But the idea takes root. Grows into something that feels right.

‘You think you’d have been a better mother than me.’

‘I’d have been a more grateful one, that’s for sure,’ she bites back.

‘I love my children.’ I can’t believe she’s even questioning it.

‘You hardly saw them! They were an inconvenience, parcelled off to mine whenever you were sick of them. Who was it who taught Katie how to cook? Who got Justin away from the thieving kids at school? He’d have ended up in prison if it hadn’t have been for me!’

‘You said you were happy to have them.’

‘Because they needed me! What else did they have? A mother who was constantly working, constantly moaning, constantly crying.’

‘That’s not fair, Melissa.’

‘It’s the truth, whether you like it or not.’

Next to me, Katie is silent. I look at her and see she is shaking, her face completely devoid of colour. Melissa straightens. She moves to sit at the swivel chair by her desk, and switches on the computer.

‘Let us go, Melissa.’

She laughs. ‘Oh, come on, Zoe, you’re not that stupid. You know about the website now; you know what I’ve done. I can’t just let you go.’

‘So leave us here!’ I cry, suddenly realising there’s another way. ‘You go, now. Lock us in. We won’t know where you’ve gone, and we won’t tell the police anything you’ve told us. You could delete everything from your computer!’ I’m aware I’m sounding hysterical. I stand up, unsure even as I do so exactly what I’m planning to do.

‘Sit down.’

I can’t feel my legs, yet they move towards Melissa on autopilot.

‘Sit down!’

‘Mum!’

It happens so fast I don’t have time to react. Melissa gets out of her chair and slams in to me, knocking us both to the ground and landing on top of me, pinning me to the floor. Her left fist is clutching my hair, forcing my chin up, and her right hand is holding the knife to my throat.

‘I’m getting tired of this, Zoe.’

‘Get off her!’ Katie screams, pulling at Melissa’s jacket and placing a well-aimed kick to her stomach. She scarcely registers it and I feel the blade of her knife pressing against my skin.

‘Katie.’ My voice is barely a whisper. ‘Stop.’ She hesitates, then backs away, shaking so hard I can hear her teeth chattering. There’s a stinging sensation at my throat.

‘Mum, you’re bleeding!’

I feel the wetness trickle down the side of my neck.

‘Are you going to do as you’re told?’

I nod my head, the tiny movement causing another trickle of blood to escape from the cut on my throat.

‘Excellent.’ Melissa gets up. She brushes her knees, then pulls a tissue from her pocket and wipes the blade of the knife carefully. ‘Now, sit down.’

I do as I’m told. Melissa returns to her desk. She taps the keyboard and I see the familiar background to the findtheone website. Melissa enters a user name and password, but the site looks different; and I realise she has logged in as an administrator. She minimises the window then opens a new one, making several swift keystrokes. I see an Underground platform. It’s not busy; there are about a dozen people standing up, and a woman with a wheeled shopper sitting on a bench. I think at first we’re looking at a photograph, then the woman with the wheeled shopper stands up and begins walking along the platform.

‘Is that CCTV?’

‘Yes. I can’t take the credit for the cameras, just the redirection of the footage. I contemplated installing my own cameras but it would have meant restricting myself to just a couple of Underground lines. This way we can see the whole network. This is the Jubilee line.’ Another flurry of keystrokes, and the image changes to a different platform, a handful of people waiting for a train. ‘I can’t get the whole network, and annoyingly there’s no opportunity to control the direction of the cameras – I can only see what the operators can see. But it’s made the whole operation far easier, not to mention more interesting.’

‘What do you mean?’ Katie says.

‘Before I had the network I never knew what happened to the women. I had to take them off the website once their profiles were sold, as well as check they hadn’t changed jobs, or altered their route to work. Sometimes it would be days before I’d realise a woman was wearing a new coat. That’s not good for business. CCTV means I can watch them whenever I want. It means I get to see what happens to them.’

She continues tapping at the keyboard, before pressing enter with an exaggerated flourish. A slow smile spreads across her face as she turns to us.

‘Now, how about we play a little game?’