Commuter Rachel Watson (Emily Blunt) catches daily glimpses of a seemingly perfect couple, Scott and Megan, from the window of her train. One day, Watson witnesses something shocking unfold in the backyard of the strangers' home. Rachel tells the authorities what she thinks she saw after learning that Megan is now missing and feared dead. Unable to trust her own memory, the troubled woman begins her own investigation, while police suspect that Rachel may have crossed a dangerous line.