Rachel Watson is an alcoholic woman in her mid-thirties. She recently got divorced from her now-ex-husband Tom, and she’s also lost her home and her job amid debilitating struggles with infertility, depression, and alcoholism. Rachel occupies herself by riding the train back and forth between London and her suburb of Ashbury each day because it offers a view of the back garden at her old home, 23 Blenheim Road in the suburb of Witney. Rachel enjoys spying on a gorgeous young couple who live a few doors down at 15 Blenheim Road and imagining what their lives must be like. One morning, Rachel spies the woman who lives in number 15 kissing another man on her terrace. Rachel becomes furious at the woman’s infidelity yet intrigued by what could be happening beneath the surface of her picture-perfect life.
As the novel progresses, Hawkins alternates between Rachel’s narration (which takes place over the summer of 2013 after the woman at number 15, Megan Hipwell, goes missing), and the narration of Megan herself. The voice of Tom’s new wife, Anna, is also interspersed. Megan’s narration begins in the spring of 2012 and catalogues the tumultuous year leading up to her disappearance. During this year, Megan experiences marital problems with her possessive but passionate husband, Scott, and she visits a therapist named Dr. Kamal Abdic who helps her unspool the threads of her dark past. She also an affair with Tom Watson, whom she meets through babysitting his daughter Evie.
Megan quits her nanny job at the end of the summer of 2012, uncomfortable with the responsibility of caring for a baby each day and increasingly nervous that Anna will discover her affair with Tom. Megan struggles to parse her romantic and sexual feelings for Tom and Scott—and all the while, old traumas create a nagging sense of restlessness within her. During this time, she confides in Dr. Abdic about her painful past—notably about a relationship during her teen years with Mac (a friend of her deceased brother Ben), and the child they had (and lost) together, Libby. Megan gradually develops romantic feelings for Dr. Abdic, and when Megan discovers that she is pregnant and realizes that the baby could be either Scott’s or Tom’s, she asks Dr. Abdic for advice about what to do. He assures her that she has come far enough in life to be a good mother. Emboldened by Abdic’s faith in her, Megan reaches out to Tom to tell him about the baby. When he encourages her to get an abortion, an incensed Megan starts screaming at Tom, threatening to make him pay for the child. Tom picks up a rock and bludgeons Megan to death.
Meanwhile, in 2013, Rachel wakes one Sunday morning to discover that she is covered in cuts, bruises, and dirt. She cannot remember what happened the night before—except that she went to Blenheim Road and had a fight with Tom and Anna. As Rachel tries to piece together the events of Saturday night, she is shocked to learn from the news that Megan Hipwell—the woman she has been observing from the train—is missing. Rachel feels certain, because she was in Witney, that she must have seen something important. Rachel obsessively reads articles about Megan’s disappearance—none of which mention the fact that Megan was having an affair. Rachel eases up on her drinking as she becomes determined to investigate Megan’s disappearance on her own. She returns to Witney to visit the underpass beneath the train station, remembering that she was there on the night Megan disappeared. While in the underpass, she has a sensorially vivid flashbacks of pain, fear, and violence.
When Rachel returns home, she finds that the police and a detective by the name of Gaskill are waiting for her. Tom and Anna have told them that Rachel was in Witney the night of Megan’s disappearance—and that Rachel recently snuck into their house, carrying Evie off into the garden. Rachel insists she would never harm Tom, Anna, or Evie, and that she doesn’t recall seeing Megan on Saturday. The police urge Rachel to stay away from Blenheim Road. Feeling guilty, Rachel goes to the police station the following day to inform Gaskill and another detective, Riley, about Megan’s affair. During the interview, Rachel is shocked to learn that Megan worked as a nanny for Tom and Anna. Afterward, she gets in touch with Scott via email, feeling that she needs to inform him about Megan’s affair. When the police call Rachel in for more questioning, she gets drunk (breaking her sobriety) and blacks out again one evening, sending more emails to Scott—and Tom—in the process. Rachel is surprised when Scott emails her back and wants to meet up to discuss Megan.
Rachel meets with Scott and describes having seen Megan kiss another man from the train. Scott shows Rachel a picture of Megan’s therapist, Dr. Kamal Abdic, and asks if Abdic is the man Rachel saw. She confirms that he was. As Scott begins confiding in Rachel about his relationship with Megan, Rachel wonders how much she can trust Scott—he is, after all, a murder suspect. The next morning on the train, Rachel spots a man reading an article about an arrest made in the Hipwell case. Rachel disembarks at Witney and hurries to Scott’s house. Scott pulls her inside and explains that Rachel doesn’t need to worry—it’s Abdic the authorities have arrested. As Rachel departs, she runs into Tom, Anna, and Evie on the street. The next day, Rachel ignores calls from Tom as she watches the news and drinks. She blacks out and falls asleep. In the morning, she discovers she placed multiple calls to Tom and Scott in the middle of the night. She calls Scott back, and Scott accuses her of lying to him—he has spoken to the police and learned that Rachel is not a friend of Megan’s and that Rachel herself was at one point a person of interest in the case.
The summer stretches on. Abdic is released due to insufficient evidence. Rachel drinks heavily and experiences nightmares about the underpass in Witney. Rachel reaches out to Tom, begging him to tell her what happened the night of Megan’s disappearance, but he is evasive. Scott reaches out to Rachel and apologetically invites her over. The two bond as they discuss Rachel’s issues with infertility and Scott’s struggles to inspire maternal instincts within Megan. Anna spots Rachel in the neighborhood and, believing Rachel is dangerous, becomes perturbed by Rachel’s frequent visits to the Hipwell house. One morning, while watching the news, Rachel learns that the body of a pregnant woman has been found in Corly Wood, near Witney—it is Megan. Rachel begins experiencing more flashbacks to the night of Megan’s disappearance: she remembers ducking from a man’s fist and seeing a woman in blue getting into a car on the other side of the underpass. Determined to recover her lost memories—and to get a read on Abdic—Rachel makes an appointment at his practice. During the session, Rachel tells Abdic about her failed marriage and her struggles with alcoholism. She tells him that she often blacks out—but that when she’s told the horrible things she said and did while drunk, she feels she couldn’t have done them.
The next morning, Rachel experiences a dream which is really a memory: a horrible fight with Tom culminating in smashed photographs and angry words. Rachel takes a walk to clear her head. At a newsstand, she sees a front-page headline about Megan being a “child killer”—authorities have discovered that Megan accidentally (or purposefully) killed her infant daughter years ago. Anna sees the headline too, and she becomes ashamed of hiring Megan to look after Evie. Anna tells Tom that she wants to leave Witney forever, but Tom shrugs her off. A few days later, when authorities recover the remains of Megan’s first child buried in a seaside town, Rachel heads over to the Hipwell house to comfort Scott. The two of them get drunk and have sex. In the morning when Rachel leaves, she runs into Anna once again. Anna lobbies Tom to call the police and get rid of Rachel once and for all, but instead, Tom asks Rachel to meet to discuss her frequent presence on Blenheim Road. Rachel insists that she’s become friendly with Scott and has no interest in disrupting Tom and Anna’s life. After another session with Abdic, Rachel returns to the underpass to try to recall more memories. Anna, having seen Rachel on the street yet again, becomes convinced that Tom and Rachel are having an affair.
Soon after, Scott asks Rachel over again. When she arrives, he drunkenly tells her that the DNA tests done on Megan’s corpse have shown that the baby she was carrying was neither his nor Abdic’s. Scott demands to know what Megan told Rachel, eventually discovering that Rachel didn’t know Megan at all. Scott attacks Rachel and imprisons her in the spare room upstairs for a short time before letting her go, after which Rachel calls the authorities and tells them what happened. They warn her again to stay away from Scott. Rachel calls Tom and begins pressing him for more details about the night of Megan’s disappearance, but he is cruel and dismissive as he orders her to stop contacting him. That night, as Rachel drifts off to sleep, she has a flashback of Tom attacking her in the underpass while the woman in blue—a woman she now believes to be Megan—looked on. Meanwhile, Anna is increasingly worried about Tom’s infidelity, so she goes through his laptop and his belongings. She finds a burner phone in his gym bag while he’s out at the pub. Later, Anna takes the phone out to the garden and listens to the outgoing voicemail message: the phone, she realizes, belonged to Megan.
Rachel begins experiencing increasingly vivid flashbacks of drunken nights when Tom attacked her. Though Tom always told Rachel that she was the instigator in all their fights, Rachel realizes that Tom was gaslighting her: for the entirety of their relationship, he was the abusive one. Rachel, concerned for Anna, travels to Tom and Anna’s house and urges Anna to pack a bag, take Evie, and leave. Anna says that she knows Tom’s a liar—but that she refuses to leave him simply because he was having an affair with the nanny. Rachel suggests that Tom killed Megan, but Anna cannot accept such an idea.
Just then, Tom comes home and confronts the two women. Rachel begs Anna to stand up to Tom, but when Tom picks up Evie and begins essentially holding her hostage, Anna is powerless. Rachel tries to run for the door, but Tom knocks her unconscious and moves her into the kitchen. As Tom tells Rachel the story of how he killed Megan, Rachel slyly reaches into a kitchen drawer and retrieves a corkscrew. She manages to run out to the garden, but Tom catches up with her. Rachel stabs Tom in the neck as a train screeches past. Anna approaches the scene, places her hands on Tom’s neck, and drives the corkscrew in further. In the aftermath of Tom’s death, Rachel takes a train journey northward toward Scotland, stopping in Megan’s hometown to visit her and Libby’s graves. Rachel remains haunted by the things that have happened to her—but free of Tom and newly sober, she is grateful to be alive.