Rachel takes the same commuter train every day, rattling over the same junctions, flashing past the same townhouses. The train stops at the same signal every day, and she sees the same couple, breakfasting on their roof terrace. Jason and Jess, as she calls them, seem so happy. Then one day Rachel sees someone new in their garden. Soon after, Rachel sees the woman she calls Jess on the news. Jess has disappeared. Through the ensuing police investigation, Rachel is drawn deeper into the lives of the couple.