A married woman is with her lover in a secluded place by the river when she sees a man run past. She comes to believe that the man has been involved in a crime. Should she go to the police and risk exposing her affair and destroying two families?

My elder daughter had just started college when I was planning the book, and that was one of the experiences that shaped my story. It's a moment of huge significance, when your child properly enters the adult world - but there's nothing to mark it. I wondered what might happen at this point in her life to a woman whose marriage has been held together by the shared project of child-rearing - how it might force her to acknowledge the fragility of that relationship.

I also wanted to write more generally about midlife and the kinds of things that preoccupy me and my friends who are at this life stage: waking in the night and thinking about death; a sense that somehow things haven't turned out quite the way you'd planned. And I wanted to explore a kind of love affair that I suspect is quite common when women are in their forties - a love affair that's perhaps less romantic and more intensely physical than when women are younger: and that's very secret, because both people are determined not to change anything in their lives. What happens if the lovers in such a relationship are faced with exposure, and everything starts to unravel?

Another inspiration was the landscape near where I live. My house is on a banal suburban street, but nearby there's the river Thames, and beside it a narrow belt of wild ground. It's a contradictory landscape - beautiful, but rather sinister: you rapidly start feeling uneasy if you're alone there. I liked this as a setting for a love story: I wanted Ginnie and Will to enter a separate world, a territory that's quite apart from their everyday lives, and where the normal rules don't apply.

Published by Mira, UK, January 2011