Rachel takes the same commuter train every morning and night. Every day she rattles down the track, flashes past a stretch of cozy suburban homes, and stops at the signal that allows her to daily watch the same couple breakfasting on their deck. She's even started to feel like she knows them. Jess and Jason, she calls them. Their life—as she sees it—is perfect. Not unlike the life she recently lost. And then she sees something shocking. It's only a minute until the train moves on, but it's enough. Now everything's changed. Unable to keep it to herself, Rachel goes to the police. But is she really as unreliable as they say? Soon she is deeply entangled not only in the investigation but in the lives of everyone involved. Has she done more harm than good?

Rachel Watson, a recent divorcee, is struggling with alcoholism and personal demons. Her daily train commute allows her to observe the seemingly perfect lives of others, especially a couple, Jess and Jason, whose lives she fantasizes about. One day, Rachel witnesses something shocking and becomes embroiled in a missing persons investigation that unravels her life and the lives of those around her. As secrets are revealed, Rachel must confront her past, unravel the truth, and ultimately decide where her loyalties lie.

Rachel Watson is a 33-year-old woman who is coping with the aftermath of her divorce from her husband, Tom. Living in a shabby flat in London, Rachel struggles with alcoholism and the inability to move on—her life having been effectively shattered by Tom's infidelity, which led to her losing both her husband and her home. Every day, Rachel takes the train into London for work, though she has actually been fired from her job and does not have the heart to admit it to anyone.

During her commute, Rachel finds herself fixated on the houses she passes, particularly one that belongs to a couple, whom she has named Jess and Jason, creating a fantasy life for them. She imagines their lives are perfect and longs for the happiness she perceives from observing them. The couple she watches is actually named Megan and Scott Hipwell. Rachel's obsession intensifies on the day she sees Megan in a compromising position through the train window. This observation plants a seed of foreboding in Rachel's mind.

The plot thickens when Rachel learns that Megan goes missing, and Rachel feels compelled to involve herself in the investigation. She approaches the police claiming she has seen something important, but they dismiss her information due to her drunken state during their meeting. However, Rachel's memories of the event, intertwined with her blackouts, leave her feeling increasingly uncertain about what she did or didn’t see.

The narrative is interlaced with the perspectives of Rachel, Megan, and Anna (Tom's new wife), giving readers insights into their complex lives. Megan, who is struggling with her own demons, including a past that involves being a baby-sitter for a child who died tragically, seeks to escape her own reality. We learn that Megan is living a dual life, filled with secrets, and is found to be involved in an affair with another man, which intensifies the intrigue.

Rachel's desire to help the police ties into her deep-seated loneliness. She embarks on her own investigation into Megan's disappearance as she becomes determined to prove her worth and reclaim her life. The comparison of Rachel’s and Megan’s struggles reveals how fragile life can be, showing how each woman copes with their situation differently.

As the days pass, we learn about Tom and Anna’s perspective, particularly Anna's disdain for Rachel and her continued interference in their lives. Rachel grapples with her memories and the rapidly unraveling truth. In flashbacks, it becomes clear that Megan had been sexually assaulted, and her relationships were fraught with tension that her past couldn’t allow her to escape.

Soon, evidence mounts against Rachel, painting her as a potential suspect in Megan's case, even beyond her perception of the events leading up to the disappearance. Everything comes to a climax when Rachel finally pieces together the fragmented memories of that fateful night. It's revealed that she had unintentionally witnessed the murder of Megan due to a conniving plan involving Tom, Anna, and their shared secret. This revelation comes with a horrific realization that pushes Rachel over the edge, causing her to confront her own unacceptable behavior as well as her role in Megan’s death.

In a gripping finale, Rachel visits the scene of the crime after a chaotic effort to collect her thoughts and make sense of the web of lies she has been trapped in. There, she confronts Megan’s killer and finds the strength to finally assert herself. The denouement reveals the implications of the characters’ choices—Tom’s manipulations lead to the ultimate unraveling of not just Megan’s life but Rachel’s as well. By the end of the novel, the lives of each character are irrevocably changed, and the cyclical nature of trauma, trust, and deception becomes apparent. Rachel ultimately embarks on a journey of self-discovery and empowerment, as the story closes with the lingering impact of both Megan’s tragic fate and Rachel’s long-standing struggles with her identity.

"The Girl on the Train" explores themes of memory, identity, and the complexities of human relationships. It delves into the idea of the unreliable narrator, using Rachel’s alcoholism and fragmented memories to create suspense and uncertainty. The novel highlights the dangers of deception, both self-imposed and from others, and examines the contrast between perceived perfection and hidden truths. Through the intertwining stories of Rachel, Megan, and Anna, Paula Hawkins crafts a gripping psychological thriller that keeps readers questioning until the very end.