Note: Before I Go to Sleep is divided into three parts. This summary subdivides the parts into chapters to make the text easier to navigate.

Christine Lucas wakes in an unfamiliar bedroom, believing she is in her twenties. Seeing a man lying next to her, she assumes she has had a one-night stand with an older, married man. In the bathroom, Christine is shocked to see a wedding ring on her finger. She looks in the mirror and appears to have aged at least 20 years. Beside the mirror are labeled photographs of herself and the stranger in the bed. The notes inform her that the man is her husband, Ben. Christine confronts the man in the bedroom, demanding to know what is happening. He tells her they have been married for 22 years, and Christine is 47. Christine’s memory is affected by a head injury she suffered when she was 29. Ben assures Christine that he loves her and will take care of her.

Ben shows Christine around their North London home. He gives her a scrapbook containing more photographs of her life. Ben explains that they go through this process most mornings as Christine’s memories are erased when she sleeps.