After Rachel Watson is divorced by her husband, she hates riding to work on the train each morning and seeing her ex-husband living in their old house with his new wife. To distract herself, Rachel focuses on their happily married neighbors, Megan and Scott Hipwell instead. One day, as she passes on the train, she sees something shocking. The next morning, Rachel wakes up with an awful hangover, no memory of yesterday, and several wounds. She has a vague feeling that something bad is going on, but she cannot remember what until she sees on the news that Megan is missing.