Reeling from her recent divorce, Rachel Watson (Emily Blunt: Into the Woods) spends her daily commute fantasising about the seemingly perfect couple who live in a house that she passes on the train. That is until she sees something shocking happen there, and becomes entangled in the mystery that unfolds. Driven by intrigue and obsession, Rachel has a chace to become a part of the lives she's only watched from afar, but to what lengths will she go to uncover the truth?