Rachel (Emily Blunt) is a divorced woman who is stalking her ex-husband, Tom (Justin Theroux of THE LEFTOVERS). It's clear she has a lot of problems and through flashbacks we learn she's an alcoholic. During the days she rides the train out to Long Island, to her old neighborhood where her husband now lives with his new wife. She also begins to notice a neighbor from the train, a young couple whose relationship looks ideal. However, when she discovers that the young woman is killed she can't help but try to figure out who killed her. She thinks that by somehow solving this crime she can somehow fix her own life.
This is one of those thrillers that begins as one thing and, by the end, transforms into a completely different film with some surprising revelations. It's like a modern-day Hitchcock film. Highly recommended.