Recovering alcoholic Rachel Watson is trapped in a monotonous routine, taking the train into New York City each day after the collapse of her marriage and career. From her train window, she becomes obsessed with the lives of her ex-husband, Tom, his new wife, Anna, and their neighbors, Scott and Megan Hipwell. Once a nanny for Tom and Anna, Megan recently left her job, adding another layer of complexity to Rachel's tumultuous situation.
During her marriage, Rachel battled depression stemming from her infertility, which spiraled into a drinking habit that triggered troubling blackouts and self-destructive antics. One fateful day, at a barbecue hosted by Tom's boss, her drunken behavior resulted in Tom's dismissal. In her inebriated state, Rachel frequently harasses Tom and Anna, though she has little recollection of these encounters once sober.
One afternoon, Rachel experiences a jarring moment as she spots Megan kissing a stranger. Consumed by rage, she confronts Megan, but the next thing she remembers is waking up hours later in her own bed, mysteriously covered in blood. As Megan is reported missing, Detective Riley questions Rachel due to her presence in the area around the time of the incident. In a misguided move, Rachel pretends to be Megan's friend when she contacts Scott, which leads to an alarming revelation—a picture of Dr. Kamal Abdic, Megan's psychiatrist, who Rachel identifies as the man she saw with Megan. This turns the investigation's focus onto Dr. Abdic, who then claims that Scott had been emotionally abusive towards Megan, raising further suspicions.
Determined to uncover the truth, Rachel arranges to meet Abdic but ends up revealing her own emotional struggles instead. Abdic recounts a traumatic memory from a session with Megan, revealing that she had a baby at a young age who tragically drowned in the bathtub—a burden Megan could never overcome. As the investigation intensifies, Megan's body is discovered, revealing that she was pregnant; however, neither Scott nor Abdic turns out to be the father. Tensions mount as Scott confronts Rachel for deceiving him, ultimately leading to Abdic becoming the next prime suspect.
As Rachel attempts to report her concerns about Scott's aggression, the police dismiss her claims, stating he has been cleared due to alibi footage. While riding the train, Rachel encounters Martha, Tom's former boss's wife, and feels compelled to apologize for her earlier antics at the barbecue. To her surprise, Martha clarifies that she was merely ill during the event, exposing the lies Tom had fed Rachel during her drinking days. This revelation makes Rachel recognize the abusive behavior she endured from Tom during her blackouts.
As Anna begins to suspect Tom's fidelity, she discovers a hidden cell phone belonging to Megan, which contains a voicemail greeting that further complicates matters. Now sober, Rachel recollects the events prior to Megan’s disappearance, realizing she witnessed Tom hit Megan when she tried to confront them about their meeting.
Fueled by anger and determination, Rachel warns Anna of Tom's danger. When both confront him, an explosive confrontation ensues, with Tom trying to force Rachel back into drinking. As the confrontation escalates, he attempts to strangle Rachel, but in a fierce struggle, she retrieves a corkscrew and stabs him. In a moment of mutual vengeance, Anna joins in, ultimately causing Tom's death.
Following the incident, both women corroborate their stories to Detective Riley, asserting that their actions were in self-defense after Tom admitted to murdering Megan. In the aftermath, Rachel pays her respects at Megan's grave, reflecting on their intertwined lives. She then chooses to sit on a different side of the train, embracing the hope of a new beginning as she moves forward.
