Rachel Watson is a troubled woman, struggling with alcoholism and the breakdown of her marriage to Tom, who left her for another woman named Anna. Rachel's daily train commute takes her past her old house where Tom and Anna now live with their baby. She also becomes fixated on the seemingly perfect couple, Megan and Scott Hipwell, who live a few houses down. Her life becomes entangled with theirs as she projects her longing and losses onto them. One day, as the train passes by, she witnesses something shocking in the Hipwells' backyard. The event consumes her, and she involves herself in the lives of strangers under the influence of her spiraling obsession and alcohol-induced blackouts.

The following day, Megan Hipwell goes missing, and Rachel becomes deeply involved in the mystery, despite her unreliable memory and unstable state. Her interference leads to her being questioned by the police and becoming an unreliable witness due to her frequent memory lapses. Rachel reaches out to Scott with what she believes she knows about Megan, establishing a precarious relationship with him. As she struggles to recall important details, her path crosses with Dr. Kamal Abdic, Megan's therapist, which only complicates the situation further. The intertwining of Rachel, Megan, and Anna's lives reveals a complex web of secrets and emotions, each holding pieces to the puzzle of Megan's disappearance.