Rachel Watson is a divorcee who takes the train to New York city every day. The train passes by her old house where she used to live with her ex-husband. As she attempts to not focus on her pain, she starts watching a couple a few houses down -- Megan and Scott Hipwell. She creates a wonderful dream life for them in her head, about how they are a perfect happy family. And then one day, as the train passes, she witnesses something that changes everything she thought about the couple. The next day, she wakes up with a horrible hangover, various wounds and bruises, and no memory of the night before. TV reports deliver the shocking news that Megan is missing, and Rachel wonders if she is somehow connected to her disappearance.