You don't know her, but she knows you. After her divorce, Rachel is devastated. Every day she takes the train to work, and every time the commuter train stops at the same section of the route. In a house on this railway line, Rachel observes a couple. The two seem to lead a perfect life, which Rachel imagines in detail in her daydreams. Until one day she makes a shocking observation and soon afterwards becomes more and more entangled in a dangerous web of lies and fantasies ...