'Gen?' Art appears from the kitchen, his iPhone in his hand. He smiles at me and suddenly all my fears about him being involved and the drama of everything that happened earlier with Lorcan feel like a dream.

This is my reality. My home. My husband. There is no way Art knows where I've been. It would show in his eyes if he did.

Art puts his phone down on the hall table and strides towards me. 'I was worried you'd get stuck in town,' he says. 'Apparently the snow's starting to settle and they're predicting transport chaos. As bloody usual.' Art envelops me in a hug. 'Jesus you're freezing.' He keeps his arm around my shoulders, walking me into the kitchen. He sits me down at the table and puts the kettle on, eager to warm me up with a cup of tea, then sits down beside me. 'I'm so sorry we had that row last night, Gen.' His voice drops to a whisper. 'It's just really hard when you don't trust me.'

'I know.' And in that moment I do, absolutely, see how unfair I've been on Art. Whatever Dr Rodriguez did, I can't believe Art knew, because it's impossible to get my head around the idea that he could have colluded in keeping Beth from me all those years ago and through all the years since. In that moment I decide that I can't yet tell him I tracked Rodriguez down. He will only take it as another sign of mistrust, not to mention reinforce his belief that I'm obsessed with chasing a dream. After all, I'm still operating on the basis of hunches and overheard conversations.

Okay, so Rodriguez mentioned having been paid money, but he didn't say for what. And yes, both the others involved in the delivery Mary Duncan, the nurse, and Gary Bloode, the anaesthetist have since died, as has Lucy O'Donnell. And Lorcan and I were not the only ones to break into Rodriguez's house.

But none of it proves Beth is alive. And Art surely doesn't have anything to do with any of it. Even if he was capable of sustaining such a lie, why would he do so? After all, what possible reason could my husband, who so badly wants a child, have for pretending that our daughter died?

I feel guilty, not telling him where I've been and what I've done, but it's easier than opening the can of worms my trip to Oxford would become. Maybe the memory stick Lorcan and I found will provide some sort of proof of what Rodriguez has done. Now that I'm back at home, I half-wish I hadn't let it out of my sight. I try to tell myself a few hours won't make any difference. I'll talk to Art when I've seen what's on the memory stick . . . when, hopefully, there's something more concrete to show him.

Art asks about my evening out and I answer as vaguely as I can.

Lovely to see the girls . . . took ages to get back.

Art swallows it all which leaves me feeling even more guilty. While he makes my cup of tea he tells me about the ICSI stats, just the topline findings. He thinks we should definitely give it a go. Not wanting to argue, I say I'll think about it. I give him a hug as he sits down again. He smells of the office and himself his own particular Art smell that's as comforting to me as home.

'What's that for?' he says, pleased.

'Nothing, just glad we're not arguing. How was your day?'

He tells me about today's meeting at 10 Downing Street. 'The PM was really impressed with our model for incentivizing profit-making.' Art beams like a little boy. 'We talked about a couple of their policies. He was pumping me for information, Gen. Practical stuff he can use in draft legislation. Afterwards Sandrine told me he never reacts like that . . . that I should seriously consider a career in politics myself.'

'Wow.' In spite of all the anxieties swirling around my head, I'm genuinely impressed. 'I can't imagine you as a politician.'

'Me neither.' Art grins. 'All that "having to please the electorate".'

His mobile sounds and he takes the call. I head upstairs and run a bath. I'm going over everything I've found out today. Rodriguez has definitely covered something up. But what?

I'm out of my clothes, about to step into the steamy water, when the doorbell rings. I hesitate, my foot poised above the side of the bath, wondering who the caller is. Could it be Lorcan? Maybe he's already seen his son and found out what's on the memory stick. Maybe it contains some kind of confession from Rodriguez . . . or perhaps copies of falsified documents? If Rodriguez passed Beth on to another couple, there might even be a fake birth certificate. My heart thuds as I drag on my long T-shirt and open the bathroom door. I can hear Art talking downstairs, but his voice is too low to make out what he is saying.

A woman's voice answers. Thoughts of Lorcan fly out of my head. Who is at the door? I'm at the top of the stairs now. Art hasn't let the woman inside the house, but she's still speaking. His body hides her from view. His back is tensed, like he's angry. My stomach twists into knots as I pad softly down the stairs.

Art is talking again, his voice a fierce, low hiss. I can't hear what he's saying. Who on earth is he talking to?

I'm almost down and the stairs creak under my feet.

Art turns, half-shutting the door on the woman on the other side. Why doesn't he want me to see who it is?

'Art?' I scuttle down the rest of the stairs, the knots in my guts turning and twisting. 'Who's there?'

A flash of anger crosses Art's features, then his face settles with a practised calm. Panic rises inside me. And then Art steps back.

Charlotte West is on the doorstep. I stare at her, too shocked to speak. She stares back, her expression both guilty and resentful. In less than a second I've registered that she's still got that fringe and the Orla Kiely bag. And that she's also wearing a soft blue wool hat, almost identical in colour to the beanie I was wearing when I bumped into her the other day. A chill snakes down my spine.

'Charlotte?' I say. 'What are you doing here?' I glance at Art. His face is thunderous.

'I was just passing . . .' Charlotte's voice is high and fake. 'Remember when I saw you around here before? I'm visiting that same friend again.'

'How did you know where I live?' I walk to the front door, tugging my T-shirt further down my thighs, self-conscious.

Charlotte shrugs. 'You mentioned it the other day,' she says, 'when I bumped into you around the corner.'

I search my memory. I might have given her a street name, but surely I never told her a house number.

'I recognized the car.' Charlotte points to Art's Mercedes parked outside. 'I've seen Art pick you up from the Art and Media Institute in it.'

'Oh.' It's true Art has, once or twice in the past few months, come to meet me from work in the car, but I can't believe Charlotte would have seen us and remembered the car's make and licence number.

'This, er, lady, says she's one of your students,' Art says, tight-lipped.

'Your husband is even better-looking than on TV.' Charlotte's carefully made-up face softens as she smiles at me. Her hand flutters over her fringe and her blue wool hat. 'Gosh, I'm so sorry to have bothered you. I wasn't thinking how late it is.' She steps away from the door.

I'm still staring at her. She's lying. She knew exactly what time it was. She glances at Art and I see the look of adoration in her eyes. What the hell is going on here?

Charlotte turns away and heads down the front path. Art shuts the door before she's even reached the pavement.

'Bloody woman,' he says.

'I don't understand,' I say, struggling to make sense of what has just happened. 'Do you know her?'

Art shakes his head. 'No, but it's obvious, isn't it? She's seen me on The Trials. Tracked me down. I can't believe she's one of your students . . .' He shakes his head again. 'God, the lengths some people will go to . . .' Muttering, he marches off to the kitchen.

I stare after him. Is that true? Is Charlotte West mimicking my hair and my accessories simply in order to get close to Art? I know Art has female admirers who've seen him on TV, but if Charlotte was only interested in my husband, why come to my writing class does she think she can somehow get to him through me? And, if Art has really never met her before, why did he sound so angry when he was talking to her on the doorstep? I head slowly back upstairs to the bathroom. Surely there's no way Charlotte West is somehow involved in all this, is there?

My mind goes back to the memory stick. I'm close now to finding out the truth, I know I am. I step into the bath, the water now lukewarm. As I turn on the hot tap, a new panic rises. Suppose Lorcan loses the stick? Suppose his son damages it while attempting to decrypt it? I force myself to calm down, taking deep breaths as fear threatens to consume me. I can't allow myself to imagine endless disasters. Tomorrow there will be answers. I have to believe that.

My phone beeps while I'm in the bath. It's Lorcan. His text contains his Hampstead address and reads: Cal coming over tomorrow morning. See you for lunch? Lx I text back that I'll come round after teaching my class.

Tomorrow, there will be answers.

The next morning Art's gone when I wake again. It's all I can do not to cancel today's class. The last thing I want to do right now is stand up in front of people and bang on about character development. But that would mean leaving Sami and the others trying to find a replacement teacher two days running plus the Wednesday class was cancelled last week by the Institute so I drag myself out of bed and go into town. I sleepwalk through it, relying on the fact that I've led this session a million times before. We're looking at characterization. I bring in a passage from Vikram Seth's An Equal Music and ask the class in groups to identify the core traits of the main characters as they are introduced to the reader. I leave them for a while after this to write biographies for their own characters. The whole time my mind is on the memory stick, wondering about the information it holds.

As I'm leaving the college my phone beeps. It's Lorcan.

File decrypted. Come asap.

Anxiety twists in my stomach. Why doesn't he say what the file contains?

I'm on the verge of calling him, then I realize I can't have that conversation in public.

I send a text back saying I'll be with him in half an hour. The wait is agonizing, and yet part of me doesn't want it to end. What on earth has he found? For once, I reject the bus as too slow and head for the nearest underground station. I hate the stale smell on the platform, the way the tunnel seems to press in on all sides. I feel spooked, too, startled by the rustling of a discarded plastic bag behind me as I wait for my train. I keep imagining I'm being watched, but when I turn to look over my shoulder there's no one there. I try and shake off the sensation, but it persists throughout the tube ride, and is still with me, oppressive and unsettling, as I come out of Hampstead station, walk down the high street and turn onto a quiet Victorian terrace.

I look around again. There's no one in sight. Just a couple of giggling schoolgirls in short skirts, hunched over a phone.

A minute later I'm ringing Lorcan's doorbell. He's already told me that he has leased out his own house for the duration of his Ireland contract and is living in a rented flat one of the many Victorian conversions in the area.

He looks serious as he opens the door, but turns away immediately and leads me inside without speaking. I follow him up the stairs to his first-floor flat. I get a brief glimpse of cream walls and grey carpets as he leads me into a smartly furnished living room complete with squashy couch, brown leather armchair and glass-top coffee table.

A gangly teenage boy is standing by the table. His gaze is fixed on the large-screen TV in the corner, which is playing BBC News with the sound muted.

He turns around as I walk in and offers me a shy smile. He doesn't look much like Lorcan. Darker in colouring, and with a thinner face and close-set brown eyes. He shifts awkwardly from foot to foot.

'Geniver, this is Cal,' Lorcan says.

'Hi.' I smile and raise my hand in a half-wave.

'Hi,' he says and blushes.

Poor boy. Tall and skinny, with arms and legs that don't quite seem to fit his body, he has that awkward air I remember from my own teenage years, when you know you're supposed to be able to talk with adults, yet you're not quite sure how to do it.

I'm already aware, from our conversation in the car, that Cal is fourteen, but to me he seems far younger. He picks up a ruck-sack and heads to the door.

'All right, man?' Lorcan says. 'I'll see you later.'

They talk quietly as Cal leaves the room and heads for the stairs. As his footsteps disappear downstairs I spot Lorcan's laptop on the table. It's closed, but the memory stick is inserted.

Heart racing, I walk over and turn the computer around to face me. Lorcan comes back in and stands close as I lift the lid. The screen flickers into life. A small window is open. A Real Player file.

'Is that what was on the memory stick?' I say. 'A video recording?'

'Yes, CCTV footage.' Lorcan's voice cracks as he speaks. 'I can't . . . I haven't taken in what it . . . God, I'm not sure if I . . . well, you better see for yourself.'

He leans across me to press a key on the computer, then steps back as the film begins to play.

As I watch my mouth falls open in horror, and all the blood seems to drain out of me, because here, surely, is the proof I've been waiting for.

The best and the worst news there could be.

CHAPTER THIRTEEN.

The film finishes. I'm dimly aware of Lorcan's hand on my shoulder, but it's like he can't really reach me. Like I'm shut up in my head where the world is imploding.

'Play it again.'

Lorcan reaches past me and presses the keyboard. The film fizzes into life once more.

It's in black-and-white, like the CCTV footage you'd see on Crimewatch. At first all it shows is an empty concrete corridor with a fire door at one end. And then a man walks into view. Art. He turns, facing the camera, his eyes on whoever is walking towards him. Another second and she appears: a black woman in a nurse's uniform. As soon as I see her, I remember her. Not just from the photo Lucy showed me, but from all those years ago. It's Mary Duncan, the nurse from my C-section. She is holding something wrapped in a blanket. Her mouth moves. She is talking. Art is listening, nodding.

Art takes a step towards the fire door. There's a carpark sign with the Fair Angel logo just below it, then the words 'Parking Restrictions Apply'. Mary follows Art to the fire door. Art is speaking now. Then he looks down, at whatever Mary is holding. And in this moment, before I see her, I know she is there. Beth.

Everything inside me pulls towards the screen as Mary turns and offers the bundle in her arms to Art. I'm powerless, watching, following the movement, knowing what I am about to see.

Wrapped tightly in a blanket, just her tiny, perfect face peeking out, is my baby.

Art takes her. He doesn't look at her face but I'm staring at it . . . drinking it in . . . a tiny, scrunched-up oval with big eyes and an unmistakable look of Art about her. She blinks, her mouth opening as if she's about to cry, as Mary reaches for the fire door and opens it into the darkness of the Fair Angel car park.

Art gives a brisk nod, then turns away, still holding our baby. He walks through the fire door and is swallowed up by the darkness. Mary closes the door behind him carefully, then walks away, along the corridor, out of sight.

The film fizzles out.

I stare at the screen. For a second I have this stupid feeling that Beth is trapped inside it and I have to resist the urge to pick up the laptop and hold it.

'Are you all right?'

I've completely forgotten Lorcan standing beside me.

I shake my head, unable to speak. My legs are trembling. I let myself slide into the chair by the table and hug my arms around my chest.

'Gen?' Lorcan puts his hand on my shoulder. I bow my head.

'Gen, please say something.' Lorcan sounds genuinely frightened.

I squeeze my eyes tight shut. My whole being feels like it's in freefall.

'He did it.' My own voice sounds strange hoarse and forced and somehow not really a part of me. 'Art took our baby. He did it.'

As I speak my voice breaks. A sob so painful I draw my breath in sharply.

Lorcan leans his head close to mine. He runs his hand down my arm. Half of me wants to fall into the security he offers, to give into the raw agony inside me, but the other half senses that if I let go now, I'll lose myself completely. I already have the sense of falling, tumbling over and over in a darkness from which there is no way out.

'It means she might be alive, Gen.' Lorcan's soft whisper becomes a rope to hold onto.

I grasp it eagerly. As I open my eyes, Lorcan releases me. He stands, leaning against the wall of his living room.

Reality floods back and with it a raging fury. Of two things I am sure: One: Art has betrayed me. He took our little girl and I will never forgive him.

Two: He must know where she is.

I jump up. Adrenalin is pumping through me. The tears, for now, are gone. The pain just a dull, distant ache. All I feel right here, right now, is the need to force the truth out of Art.

'Would you call me a cab, please?'

Lorcan frowns. 'Where to? D'you want me to come?'

I gaze at his concerned face and feel a wave of affection for him. I'm tempted, for a second, to say yes. Then I pull myself together. Right now my business is with my husband. For all his concern, Lorcan isn't a part of that. I barely even know him; I certainly can't let myself start relying on him.

My mind feels clean and clear, like a knife.