Plot
After losing her job and her marriage, recovering alcoholic Rachel Watson spends every day aimlessly riding a train into New York City. She fixates on the lives of her ex-husband Tom, his new wife Anna, and their neighbors, Scott and Megan Hipwell. Megan worked for Tom and Anna as a nanny for their baby Evie, but had recently quit.
During her marriage to Tom, Rachel became depressed about her infertility and developed a drinking problem, which led to blackouts and destructive behavior. At a barbecue held by Tom's boss, she drunkenly made a scene, causing Tom to get fired. Rachel often harasses Tom and Anna when she is drunk, calling them repeatedly, though she has little memory of this when she is sober.
One afternoon, Rachel spots Megan kissing a stranger. Infuriated, she goes to confront her but awakens in her own bed hours later, covered in blood. Megan is reported missing and Detective Riley questions Rachel, who was seen in the vicinity.
Rachel contacts Scott, posing as Megan's friend, to tell him about the affair. He shows her a picture of Megan's psychiatrist, Dr. Kamal Abdic, and Rachel identifies him as the man. Abdic is questioned as a suspect but reveals that Scott was emotionally abusive, and suspicion shifts to him.
Believing Abdic is involved in Megan's disappearance, Rachel books an appointment with him, but winds up discussing her own issues. He recalls a session with Megan in which she revealed she had a baby when she was very young. The baby accidentally drowned in the bathtub and Megan never forgave herself for it.
Megan is found dead and tests reveal she was pregnant, but the father was neither Scott nor Abdic. Scott enters Rachel's house and confronts her for lying about knowing Megan, directing the police toward Abdic, and leaving him as the new prime suspect. Rachel tries to report the assault to the police, believing Scott's violence suggests he may have murdered Megan. However, she learns Scott has been ruled out as a suspect as there is surveillance footage of him at a bar at the time.
On the train, Rachel sees Martha, the wife of Tom's former boss. Rachel apologizes for her behavior at the barbecue where (she was later told) she broke a platter, threw food, and insulted Martha. However, Martha reveals that Rachel just got sick and took a nap in their guest room. In fact, Tom was fired for sexually harassing co-workers.
Rachel realizes that Tom planted false memories in her head during her drinking binges. He was also physically violent with her during her blackouts, which accounts for the injuries she had when she awoke. Meanwhile, Anna suspects Tom of cheating and finds a cell phone hidden in their house; the voicemail greeting reveals that the phone belonged to Megan.
A now sober Rachel remembers on the day of Megan's disappearance, she caught Megan meeting Tom, and he hit her when she confronted them. Realizing Tom killed Megan, Rachel warns Anna.
When both women confront Tom, he becomes angry and knocks Rachel unconscious. Rachel awakens to Tom trying to kill her as Anna watches from the top of the stairs, guarding Evie. Rachel runs to the kitchen and picks up a corkscrew. After a scuffle, she stabs him in the neck with it. Anna then twists it deeper into Tom's neck, killing him. Interviewed by Riley, Rachel and Anna tell corroborating stories about killing Tom in self-defense after he admitted that he was Megan's killer. Anna admits that Rachel had been right about everything.
Later, Rachel visits Megan's tombstone at a cemetery. She then sits on the opposite side of the train, hopeful to move on.
