The story is a first person narrative told from the point of view of three women: Rachel, Anna and Megan.

Rachel Watson is a 32-year-old alcoholic who is going through a difficult time in her life. Her ex-husband, Tom, left her two years ago for his mistress. He and Anna are now married and have a daughter. However, Tom cheats on Anna by having an affair with married neighbour Megan Hipwell. As the story begins, Megan is pregnant and secretly believes Tom to be the father.

While Rachel rides the train to and from London every day, she notices a beautiful couple whose lives she envies as they look happy. She watches them on their deck and often fantasizes about their lives. Though she has never met them, she believes them to be the perfect couple. Days later, she sees a news story about a woman's disappearance and realises it is the woman - Megan. Worried that she may have done something to Megan on the day of the incident, while blacked out and drunk, Rachel begins to investigate the disappearance. She starts lying to Megan's husband, Scott, to get close to him and causes distress to both Tom and Anna with her frequent drunkenness.

Once Anna finds evidence among Tom's belongings of his affair with Megan, she and Rachel realize that Tom "lies about everything,"[citation needed] and they confront him. Tom confesses to having murdered Megan and burying her body after she told him that he might be the father of her unborn baby. Tom attacks Rachel, who kills him in self-defense.