Every day, Rachel Watson (Emily Blunt) takes the train in to work in New York, and every day the train passes by her old house. The house she lived in with her husband, who still lives there, with his new wife and child. As she attempts to not focus on her pain, she starts watching a couple who live a few houses down Megan (Haley Bennett) and Scott Hipwell (Luke Evans). 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 sees something shocking, filling her with rage. The next day, she wakes up with a horrible hangover, various wounds and bruises, and no memory of the night before. She has only a feeling: something bad happened. Then come the TV reports: Megan Hipwell is missing. Rachel becomes invested in the case and trying to find out what happened to Megan, where she is, and what exactly she herself was up to that same night Megan went missing.