Recovering from a tumultuous past marked by alcoholism and heartbreak, Rachel Watson embarks on a daily journey aboard the train into New York City, continuously haunted by her previous life—one that crumbled after the loss of her marriage to Tom. Each trip offers her a glimpse into the seemingly perfect lives of Tom’s new family, including his wife Anna and their neighbors, Scott and Megan Hipwell. Ironically, Megan once held the role of nanny for Tom and Anna's child, Evie, but has recently departed from that position.
The roots of Rachel’s struggles can be traced back to her battle with infertility and the subsequent depression that ensued, which spiraled into her addiction. The once vibrant aspects of her life rapidly devolved into chaos, climaxing at a barbecue hosted by Tom's boss where Rachel's reckless drinking led to her humiliation and, ultimately, Tom's job loss. Now, enshrouded in the fog of her inebriation, Rachel frequently invades the lives of Tom and Anna, pestering them with calls that she fails to remember once she sobers up.
On one fateful journey home, Rachel witnesses Megan sharing a passionate kiss with an unknown stranger. Driven by a mix of rage and desperation, she seeks to confront Megan but, in an intoxicated lapse, awakens in her own bed hours later, her body inexplicably smeared with blood. As the search for Megan intensifies and she is declared missing, Rachel finds herself embroiled in a police investigation led by Detective Riley, who questions her whereabouts on that fateful day.
In an effort to connect with Scott, she feigns friendship with Megan to share her observations, leading to a chilling revelation about Megan's psychiatrist, Dr. Kamal Abdic. He initially becomes a suspect but reveals that Scott's behavior was emotionally damaging to Megan, shifting suspicion once more. Rachel's concern for Megan deepens, prompting her to arrange a meeting with Abdic; however, their session soon turns into a cathartic exploration of Rachel's own unresolved traumas.
A heartbreaking discovery is revealed during a conversation; Megan once suffered the profound grief of losing her baby, a tragedy that continually haunted her. As the tension thickens, Megan's lifeless body is found, revealing she was pregnant—not with Scott's child. The circumstances surrounding her death become murky as Scott confronts Rachel for her supposed deceit, blurting out accusations which only serve to vindicate him in the eyes of law enforcement, leaving Rachel feeling increasingly cornered.
As Rachel's reality unravels further, a chance encounter with Martha, Tom’s former boss' wife, forces Rachel to confront the distorted memories induced by her drinking. Martha’s revelation shatters Rachel’s understanding of her past; she realizes that Tom had manipulated her mind during her drunken episodes, concealing his abusive actions that explain her injuries when she awoke.
In a surprising turn, Anna discovers a hidden cell phone belonging to Megan, which spirals the investigation into a deeper labyrinth of betrayal. The truth begins to crystallize for Rachel—on that dreadful day, she had witnessed a meeting between Megan and Tom, a confrontation that spiraled out of control leading to the unfortunate demise of Megan at the hands of Tom, following her refusal to terminate a pregnancy that he wished to erase.
Courageously, Rachel and Anna confront Tom, who reacts with an explosion of rage, attempting to drag Rachel back into the grips of addiction by force. In a desperate struggle for survival, Rachel wields a corkscrew against Tom, soon being joined by Anna, who helps deliver the fatal blow, avenging Megan’s untimely death. When interviewed by Detective Riley, the two women recount a cohesive tale of self-defense, embedding Tom’s cruel admission of guilt regarding Megan.
Finally, in a solemn act of remembrance, Rachel visits Megan’s grave, conveying a message of gratitude and closure. She embarks on her journey home, stepping onto the opposite side of the train with an air of hope, determined to carve a fresh path forward, free from her past burdens.
