All aboard. In July of 2013, Rachel Watson is a female on a locomotive, a.k.a. a girl on a train. She rides the commuter rail to and from London every day for work (or so she says). While on the train, Rachel engages in hardcore people-watching and conjures up a perfect fantasy life for one couple she is totally obsessed with. Rachel calls them Jason and Jess, and the train stops outside their house every morning.
This golden couple lives on the same street Rachel used to… before she failed to get pregnant, started drinking, and her husband cheated on her and divorced her. Ouch. No wonder she wants to pretend someone out there has a perfect life. Hers is far from it.
But this couple isn't perfect. Rachel soon sees Jess smooching a man who isn't her husband, and then a few days later, Jess disappears. Jess and Jason are really named Megan and Scott. When no newspapers mention the man Megan was having an affair with, Rachel decides to contact Scott and tell him that she saw his wife kissing another man.
Together, Rachel and Scott learn the man's identity: It's Dr. Kamal Abdic, Megan's therapist. Whoa. The doctor is called in for questioning by the police but he denies have an affair with Megan. Is he telling the truth? Scott and Rachel still suspect Dr. Abdic, so Rachel goes to him for therapy to see if he acts guilty. He doesn't act guilty. And he turns out to be a pretty good therapist to boot, helping Rachel confront her drinking problem and her memory loss.
Rachel remembers being in the neighborhood the night Megan went missing. The problem is that Rachel was totally drunk and doesn't remember anything beyond that. She had a blackout in the underpass, so her memory is one big black hole. Rachel's roommate, Cathy, disapproves of Rachel's drinking problem, and gets even angrier at her when she realizes that Rachel lost her job months ago. All those train rides to and from London are just her pretending she still has a job. Maybe she should be applying to jobs instead of riding the train…
So why was Rachel conveniently in the neighborhood when Megan went missing? Did she have something to do with her disappearance? Rachel's memory is completely blacked out; she has no idea.
Rachel eventually remembers something bad happening in the underpass. Although she doesn't know exactly what it was, she recalls her ex-husband, Tom, and his new wife, Anna, both being there. Rachel is unable to get over her failed marriage and often bothers Tom and Anna, who live a couple houses down from Scott and Megan. Anna, who was the other woman when Rachel and Tom were married, hates Rachel and wants her out of their lives. She wasn't very fond of Megan either, who briefly babysat for her and Tom. Wow, things are really complicated.
When Megan's body is found buried in the woods, Scott and Rachel have a brief one-night stand. Soon Anna tells the police that Rachel is a creepy desperate stalker, and word gets to Scott, who is mad at Rachel for being a creepy desperate stalker. Scott, drunk and angry, manhandles Rachel and locks her in a room. Could he have become angry enough to kill Megan? Would a man in mourning sleep with another woman days after his wife is found dead?
Rachel is still desperate to get to the bottom of these questions. She bumps into a red haired man on the train who remembers Rachel from the night Megan went missing. By talking to him, Rachel is able to piece together more details of that night: Tom wasn't with Anna that night—he was with Megan.
Just when Rachel suspects Tom may have killed Megan, Anna finds a secret phone in Tom's gym bag. The pre-paid cell phone turns out to be Megan's. Why does Tom have her phone? Rachel shows up to convince Anna that Tom is a murderer and to help Anna and her baby escape. But Anna is unable to put aside her dislike for Rachel and go with her, even though the evidence against Tom is mounting rapidly.
We flashback to the night Megan revealed to Scott that she had an affair. Her therapist had advised her to come clean, and given her a friendly kiss after their talk. That's the kiss Rachel saw and misinterpreted. It's good advice, but Scott doesn't take the news well. He assaults Megan, and she leaves, running to Tom…
Back in the present, Tom comes home and, like the super villain he is, reveals everything that happened: He was cheating on Anna with Megan, and Megan got pregnant with his baby. Tom asked her to have an abortion, not realizing Megan's history with babies. When Megan was a teenager, she had a baby, but accidentally killed it when she fell asleep in the tub. Tom's suggestion to have an abortion touched a nerve, and Megan yelled and screamed at Tom, saying she'd expose their affair. To shut her up, Tom killed her. Whoa. We thought Scott had anger issues.
After the confession, Rachel runs from Tom, but he attacks her. She stabs him in the neck with a corkscrew. Angry that Tom lied to her, too, Anna moves in and finishes the job, twisting the corkscrew in deeper and deeper. Tom dies, his lies are exposed by the police, and everyone lives happily ever after. Well, Rachel seems to be happy at least. She takes a vacation to gather her thoughts and think about what she wants next out of her life. Then she takes the train home. Or to the laundromat to get all those blood stains out of her clothing. Either destination seems likely.