The name seems familiar, but only distantly so, as if I am having to make an effort to believe that it is mine. In the photograph we are both smiling at the camera, holding hands. He is handsome, attractive, and when I look closely I can see that it is the same man I slept with, the one I left in the bed. The word Ben is written beneath it, and next to it Your husband.

I gasp, and rip it off the wall. No, I think. No! It can't be... I scan the rest of the pictures. They are all of me, and him. In one I am wearing an ugly dress and unwrapping a present, in another both of us wear matching weatherproof jackets and stand in front of a waterfall as a small dog sniffs at our feet. Next to it is a picture of me sitting beside him, sipping a glass of orange juice, wearing the dressing gown I have seen in the bedroom next door.

I step back further, until I feel cold tiles against my back. It is then I get the glimmer that I associate with memory. As my mind tries to settle on it, it flutters away, like ashes caught in a breeze, and I realize that in my life there is a then, a before, though before what I cannot say, and there is a now, and there is nothing between the two but a long, silent emptiness that has led me here, to me and him, in this house.

I go back into the bedroom. I still have the picture in my hand - the one of me and the man I had woken up with - and I hold it in front of me.

'What's going on?' I say. I am screaming; tears run down my face. The man is sitting up in bed, his eyes half closed. 'Who are you?'

'I'm your husband,' he says. His face is sleepy, without a trace of annoyance. He does not look at my naked body. 'We've been married for years.'

'What do you mean?' I say. I want to run, but there is nowhere to go. '"Married for years"? What do you mean?' He stands up. 'Here,' he says, and passes me the dressing gown, waiting while I put it on. He is wearing pyjama trousers that are too big for him, a white vest. He reminds me of my father.

'We got married in nineteen eighty-five,' he says. 'Twenty-two years ago. You - '

'What - ?' I feel the blood drain from my face, the room begin to spin. A clock ticks, somewhere in the house, and it sounds as loud as a hammer. 'But - ' He takes a step towards me. 'How - ?'

'Christine, you're forty-seven now,' he says. I look at him, this stranger who is smiling at me. I don't want to believe him, don't want even to hear what he's saying, but he carries on. 'You had an accident,' he says. 'A bad accident. You suffered head injuries. You have problems remembering things.'

Excerpted from Before I Go To Sleep by S.J. Watson. Copyright © 2011 by S.J. Watson. Excerpted by permission of Harper. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.