‘Katie!’ I scream so loudly my voice cracks, my mouth suddenly devoid of moisture. I pull at the tape, feeling the adhesive tug at the hairs on my wrists. I find a strength I didn’t know I had, and I feel the tape give a fraction. Melissa smiles.

‘I win.’ She spins her chair round to face me, folding her arms and looking thoughtfully at me. ‘But then, I was always going to.’

‘You bitch. How could you do that?’

‘I didn’t do anything. You did. You let her walk into danger; danger you knew was out there. How could you do that to your own flesh and blood?’

‘You—’ I stop. Melissa didn’t make me. She’s right; I let Katie go. It’s my fault.

I can’t look at her. There’s a pain in my chest that’s making it hard to breathe. Katie. My Katie. Who was that man? What is he doing to her?

I try to keep my voice calm. Rational. ‘You could have had children. You could have adopted; had IVF.’ I look at the screen again but the door to what I assume is some kind of cupboard or maintenance room remains stubbornly closed. Why did no one notice? There are people everywhere. I see a fluorescent jacketed Underground worker and I want so much for her to open the door; to hear Katie crying out; to do something – anything – to stop whatever is happening right now to my baby girl.

‘Neil refused.’ Melissa is staring at the screen, and I can’t see her eyes. I can’t see if there’s any emotion in them, or whether they’re as dead as her voice. ‘Said he wanted his own child, not someone else’s.’ She gives a hollow laugh. ‘Ironic, given the amount of time we spent looking after yours.’

On the screen life is continuing as usual; people are getting in each other’s way, searching for Oyster cards, rushing to catch trains. But for me, the world has stopped.

‘You lose,’ she says, as easily as if we’ve been playing cards. ‘Time to pay up.’ She picks up the knife and runs a speculative finger across the blade.

I should never have let Katie go, no matter what she said. I thought I was giving her a chance, but I was sending her into danger. Melissa would have tried to kill us, but would she have succeeded, with two of us to fight her off?

And now she’s going to kill me anyway. I feel dead inside already, and part of me wants her to finish it; to hasten the darkness that began to descend after Katie left, and which now threatens to overcome me.

Do it, Melissa. Kill me.

I catch sight of the penholder on Melissa’s desk – the one Katie made for her in woodwork – and feel a surge of rage. Katie and Justin worshipped Melissa. They saw her as a surrogate mother; someone to trust. How dare she betray us like this?

I mentally shake myself. If Katie dies, who will be there for Justin? I work my wrists again, twisting my hands in opposite directions and finding perverse pleasure in the pain which ensues. It is a distraction. My eyes are still trained on the screen as though I can make the door to that maintenance cupboard fly open through the power of thought alone.

Perhaps Katie isn’t dead. Perhaps she’s been raped, or beaten up. What will happen to her if I’m not there, at a time when she needs me most? I can’t let Melissa kill me.

Suddenly I feel cool air on a tiny patch of newly exposed skin.

I’m loosening the tape. I can get free.

I think quickly, allowing my head to sink down to my chest, in an attempt to make Melissa think I’ve given up. My thoughts are whirring. The doors are locked, and the only windows in the kitchen extension are the huge skylights, too high above my head to reach. There is only one way to stop Melissa from killing me, and that is to kill her first. The thought is so ridiculous I feel light-headed: how did I get here? How did I become the sort of woman who could kill someone?

But kill Melissa I can. And I will. My legs are too tightly strapped to even think about getting loose, which means I’m not going to be able to move fast. I’ve managed to loosen the duct tape around my wrists enough to gently pull out one hand, careful not to move my upper arms. I’m convinced my plan – such as it is – is written all over my face, so I glance at the screen, without hope of seeing Katie, but nevertheless desperate for some sign of movement from that shut door.

‘That’s odd,’ I say, too fast to consider whether I should have kept my thoughts to myself.

Melissa looks at the screen. ‘What?’

Both my hands are free now. I keep them clasped behind my back.

‘That sign’ – I nod towards the upper left-hand corner of the screen – ‘at the top of the escalator. It wasn’t there a minute ago.’ The sign is a plastic yellow folding one, warning of wet surfaces. There’s been a spillage. But when? Not while I was watching.

Melissa shrugs. ‘So someone’s put out a sign.’

‘They didn’t. It just appeared.’ I know the sign wasn’t there when Katie came up the escalator, because it would have been in front of her for a second. As for when it appeared … well, I can’t be certain, but I haven’t taken my eyes off the CCTV image for more than a few seconds since Katie disappeared, and every time I’ve seen a high-vis jacket I’ve kept my eyes trained on the wearer, desperately hoping I’ll see them walk into the room where Katie is.

There is a shadow of concern in Melissa’s eyes. She leans close to the screen. The knife is still in her right hand. Both my hands are now free, and slowly I move one of them; first to the side of the chair, then by tiny degrees down towards my legs. I keep my eyes trained on Melissa. The second she moves, I sit up straight, putting my hands behind my back, but it’s too late; she sees the movement in the corner of her eye.

Beads of sweat form on my brow and sting my eyes.

I don’t know what makes Melissa glance towards the kitchen counter, but I know instantly she’s realised what I’ve done. Her eyes flick to the knife block. Counting the knives; seeing one missing.

‘You’re not playing by the rules,’ she says.

‘Neither are you.’

I lean down and wrap my fist around the handle of the knife, feeling a sharp pain as the blade cuts my ankle on its way out of my boot.

This is it, I think. This is the only chance I’m going to get.