Excerpt from Before I Go To Sleep by S.J. Watson, plus links to reviews, author biography & more

Part One
Today

The bedroom is strange. Unfamiliar. I don't know where I am, how I came to be here. I don't know how I'm going to get home.

I have spent the night here. I was woken by a woman's voice - at first I thought she was in bed with me, but then realized she was reading the news and I was hearing a radio alarm - and when I opened my eyes I found myself here. In this room I don't recognize.

My eyes adjust and I look around in the near dark. A dressing gown hangs off the back of the wardrobe door - suitable for a woman, but someone much older than I am - and some dark-coloured trousers are folded neatly over the back of a chair at the dressing table, but I can make out little else. The alarm clock looks complicated, but I find a button and manage to silence it.

It is then that I hear a juddering intake of breath behind me and realize I am not alone. I turn round. I see an expanse of skin and dark hair, flecked with white. A man. He has his left arm outside the covers and there is a gold band on the third finger of the hand. I suppress a groan. So this one is not only old and grey, I think, but also married. Not only have I screwed a married man, but I have done so in what I am guessing is his home, in the bed he must usually share with his wife. I lie back to gather myself. I ought to be ashamed

. I wonder where the wife is. Do I need to worry about her arriving back at any moment? I imagine her standing on the other side of the room, screaming, calling me a slut. A medusa. A mass of snakes. I wonder how I will defend myself, if she does appear. The guy in the bed doesn't seem concerned, though. He has turned over and snores on.

I lie as still as possible. Usually I can remember how I get into situations like this, but not today. There must have been a party, or a trip to a bar or a club. I must have been pretty wasted. Wasted enough that I don't remember anything at all. Wasted enough to have gone home with a man with a wedding ring and hairs on his back.

I fold back the covers as gently as I can and sit on the edge of the bed. First, I need to use the bathroom. I ignore the slippers at my feet - after all, fucking the husband is one thing, but I could never wear another woman's shoes - and creep barefoot on to the landing. I am aware of my nakedness, fearful of choosing the wrong door, of stumbling on a lodger, a teenage son. Relieved, I see the bathroom door is ajar and go in, locking it behind me.

I sit, use the toilet, then flush it and turn to wash my hands. I reach for the soap, but something is wrong. At first I can't work out what it is, but then I see it. The hand gripping the soap does not look like mine. The skin is wrinkled, the nails are unpolished and bitten to the quick and, like the man in the bed I have just left, the third finger wears a plain, gold wedding ring.

I stare for a moment, then wiggle my fingers. The fingers of the hand holding the soap move also. I gasp, and the soap thuds into the sink. I look up at the mirror. The face I see looking back at me is not my own. The hair has no volume and is cut much shorter than I wear it, the skin on the cheeks and under the chin sags, the lips are thin, the mouth turned down. I cry out, a wordless gasp that would turn into a shriek of shock were I to let it, and then notice the eyes. The skin around them is lined, yes, but despite everything else I can see that they are mine. The person in the mirror is me, but I am twenty years too old. Twenty-five. More.

This isn't possible. Beginning to shake, I grip the edge of the sink. Another scream starts to rise in my chest and this one erupts as a strangled gasp. I step back, away from the mirror, and it is then that I see them. Photographs. Taped to the wall, to the mirror itself. Pictures, interspersed with yellow pieces of gummed paper, felt-tip notes, damp and curling. I choose one at random. Christine, it says, and an arrow points to a photograph of me - this new me, this old me - in which I am sitting on a bench on a quayside, next to a man.

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.