Rachel Watson is an alcoholic who divorced her husband, Tom Watson, after catching him cheating on her with their real estate agent, Anna Boyd. During their marriage, Rachel was predisposed to alcohol-induced blackouts during which she engaged in self-destructive behavior that she only later learned about from Tom, and to which she attributes the ending of their marriage, in addition to her being sterile.

Rachel shares an apartment with her friend Cathy and spends her days commuting on a train, often stalking Tom, Anna, and their newborn daughter Evie. During her trips, Rachel grows fascinated with Tom's neighbors, Scott and Megan Hipwell, a young couple in what Rachel believes is a perfect marriage.

While passing by the neighborhood one day, Rachel spots Megan kissing another man on her balcony and becomes infuriated by what she perceives as Megan's ruining her "perfect" marriage. After a drinking binge, Rachel leaves the train to confront Megan, only to black out and awaken hours later at her apartment. Rachel later finds out that Megan is missing, and is questioned by Detective Sergeant Riley, who suspects Rachel might be involved due to her recent erratic behavior.

Rachel tells the authorities what she thinks she saw. Unable to trust her own memory, the troubled Rachel begins her own investigation, while police suspect that Rachel may have crossed a dangerous line.

https://en.wikipedia.org/wiki/The_Girl_on_the_Train_(2016_film)