Trama

Rachel Watson (Emily Blunt) is an alcoholic who shares an apartment with her friend Cathy (Laura Prepon) and spends her days commuting aimlessly on a train after losing her job. From the train she observes the activities of her former husband, Tom Watson (Justin Theroux), his new wife, Anna Watson (Rebecca Ferguson), and a couple who live nearby, Scott (Luke Evans) and Megan Hipwell (Haley Bennett), whom she sees as the ideal couple.

During her marriage, having failed to become pregnant, Rachel became prone to alcohol-induced blackouts during which she engaged in self-destructive and violent behavior, including an occasion where she caused a scene at the home of her husband's boss and his wife Martha (Lisa Kudrow), leading to Tom being fired. She continues to watch Tom, Anna, and their newborn daughter Evie, causing both Tom and Anna to warn her off. Unknown to her, Megan has been working as a childminder for Anna, but has suddenly resigned without explanation.

During one train journey, Rachel spots Megan kissing a stranger on her balcony and becomes infuriated by what she perceives as Megan ruining her "perfect" marriage. After a drinking binge, Rachel leaves the train to confront Megan, only to black out and awaken hours later at her apartment, injured. Rachel later learns that Megan is missing, and is questioned by Detective Sergeant Riley (Allison Janney), who suspects Rachel might be involved due to her recent erratic behavior and her presence in the neighborhood on the evening of Megan's disappearance. Rachel suspects a fellow-passenger (Darren Goldstein) of following her off the train and sees him in flashbacks to that evening but cannot piece her memories together.

Hearing that Scott is asking the public to help him find his wife, Rachel goes to see him, pretending to be a friend of Megan's, and tells him about Megan's affair. From a photograph, she identifies Dr Kamal Abdic (Édgar Ramírez), Megan's psychiatrist, as the man on the balcony. It gradually becomes apparent that Scott is aggressive and controlling, while Megan is prone to dishonesty and sex addiction, fears becoming pregnant, and has frequent therapy sessions with Abdic. In one of her last sessions, she tells him that she had a baby girl when in her teens, whom she accidentally drowned in the bath.

Believing Abdic to be the key to Megan's disappearance, Rachel schedules an appointment with him, revealing the history of her marriage and alcoholism. Later, at a bar, she recognizes the man who followed her from the train, and confronts him. He claims to have found Rachel lying in a tunnel on his route home; he tried to help her up, but she rejected his help. Rachel returns to the tunnel and recalls seeing Megan shouting at her before she was injured.

Meanwhile, Anna becomes suspicious of Tom after Sgt. Riley suggests that he is encouraging Rachel's behavior by keeping in touch with her. Anna attempts to hack into Tom's computer, and in the process she finds a secret phone with several voice messages to another woman Tom is having an affair with. Megan is found dead and forensics determine she was pregnant, but the child was neither Scott's nor Abdic's. Scott confronts Rachel, in the process revealing that he assaulted Megan prior to her death. Rachel sees Martha on the train and apologizes for her previous behavior, but her memory of the occasion does not match Martha's version; Martha tells her that Tom was fired for having sexual relations with co-workers. Rachel realizes that Tom planted false memories in her head on several occasions following a drinking binge.

Anna finds out that Tom was exchanging phone calls with Megan. Rachel realizes that Megan had confided in Abdic about her pregnancy, leading to their encounter on the balcony. After leaving the train that day, Rachel caught Tom meeting Megan and shouted at her, mistaking her for Anna. Tom attacked Rachel before returning to Megan, who told him about the baby and refused to get an abortion. Fearing that their affair would be exposed, Tom murdered Megan.

Rachel goes to Tom's house to warn Anna, who reveals that she already knows about the affair. Tom arrives and Rachel confronts him. As Tom and Rachel argue, Anna reveals that she had found Megan's phone. Tom then carries Evie out of the room; a fearful Anna pleads for him to give her the baby while Rachel tries to dial 911. Tom knocks Rachel unconscious; when she comes round, Tom threatens her again, trying to suggest that Rachel is responsible for his actions. He tries to strangle her, watched by Anna from the top of the stairs. Rachel runs from the house, grabbing a corkscrew on her way out. Tom catches up with her in the garden, and she stabs him in the neck with the corkscrew as Anna watches from the window. Anna arrives and pushes the corkscrew deeper into Tom's neck, killing him.

Rachel and Anna are both arrested, and separately confirm to the police that Rachel killed Tom in self-defense, after he revealed himself to be Megan's killer. Anna tells the skeptical detective that Rachel was right about everything. One year later, a sobered-up Rachel is commuting to her new job on the same train. She now sits on the opposite side in a different car, leaving her old neighborhood in the past