Rachel lives with her university friend, Cathy, after she broke up with her husband and had no where to live. She has no job but hides it from Cathy. Every day, she takes the same morning and evening trains, observing the same suburban houses by the tracks. However, there is one house that she cannot wait to see each day. She names the people in it, Jess and Jason. She loves this house because it reminds her of her past, perfect life, before she and Tom divorced.
One day, she sees Jess kissing a man. Rachel is sure it isn't Jason because this man was much taller and had a different body structure. The next day, she hears the news of a woman named Megan Hipwell who went missing. From the address on the newspaper, she knows that Megan is Jess and recalls the moment she saw her kissing the man. Unable to keep it to herself, she tries to get in contact with Jason, or Scott, to tell him what she saw. She also goes to the police station and tells them about what she saw, but they end up refusing her evidence because she was drunk the night that Megan disappeared.
Scott answers her email and ask her to call him. He asks her to meet him in person. When they meet, she tells him about what she saw. He asks her if she can identify the man based on a picture and she says yes. When she sees a picture of Kamal Abdic, Megan's therapist, she says he is the man she saw with Megan. Later, Kamal is held as a suspect, but set free because of insufficient evidence. When they checked his house, there was no trace of Megan in Kamal's bedroom. Scott and Rachel still suspect Kamal though and believe he had a sexual motive.
On the day Megan went missing, Rachel remembers that she was by where they live, but she doesn't remember what happened because she was heavily drunk. As time passes, she remembers tiny details like falling on the stairs and a red-haired man helping her out. She remembers being in the underpass near the train station and her head and hands bleeding. Rachel decides to go to the therapist, Kamal, to see if he can recover her memory, and to try to find any information that will make him a suspect again. Kamal tells her that memories cannot be recovered; however, a person who has lost a memory can often remember glimpses by going back to the scene where everything happened.
The police release news that Megan is dead and, days later, that she was pregnant. Scott and Rachel continue to meet and even engage in sexual intercourse, which causes them both to feel confused and angry. Scott discovers that Rachel has been lying to him all this time - she wasn't a friend of Megan and she had never gone to her gallery. He also discovers that she has been going to Kamal Abdic and doesn't believe her when she tells him that she was trying to help. Scott drags Rachel along the stairs and she starts to bleed. Then, he locks her in a room, where she sees a framed picture of Scott and Megan shattered. She thinks that Scott murdered Megan and goes straight to the police the next day.
Eventually, Rachel recovers her entire memory of Saturday night when she was in the underpass. She remembers Tom hitting her, causing her head injury, and taking Megan away in his car. Rachel decides that she must talk to Anna about this, despite their relationship. Anna, in the meantime, finds a secret mobile phone and discovers that Tom is having an affair with someone, but she thinks it is Rachel. When Rachel tells her about Tom, she believes her. However, just as they are about to head to the police, Tom arrives and locks them in the house.
Rachel tells him about what she saw, and he at first denies it but then admits that he was having an affair with Megan. He says he was trying to end it; on Saturday night, she kept calling and threatening him. She said that if he didn't meet her somewhere she would come to his house and tell Anna everything. Tom blames Rachel for being so drunk that day and coming to their house because that upset Anna and she decided to not meet with her friends. Tom says that Megan kept shouting and cursing and he had no choice but to kill her. He buried her in the forest and ran away. The reader sees this scene from Megan's point of view; she says she was just trying to be honest with everyone and take care of her baby.
Tom now tries to kill or badly hurt Rachel. In self-defense, she kills him by putting the corkscrew in his throat. Anna calls the ambulance and tells everything to the police, clearing Rachel's name. Rachel also sees her talking to Tom before the ambulance arrives and twisting the corkscrew further into his throat. As the book ends, Rachel decides to get away from that area for a while and reclaim her sobriety and her life.