Rachel takes the same commuter train every morning and every night. Every day she rattles over the same track junctions, flashes past the same stretch of cozy suburban homes. And every day she stops at the same signal and she sees the same couple, breakfasting on their roof deck, living the perfect life that Rachel craves for herself - a lifestyle she recently lost.Then one day Rachel sees someone new in their garden, and soon after, the woman who lived there disappears. Through the ensuing police investigation, Rachel is drawn deeper and deeper into the lives of the couple. Soon, Rachel is in over her head, intricately entangled in the details of the investigation, as well as in the lives of everyone involved. Has she put others in danger? Has she done more harm than good.