Thirty-something Rachel is depressed, divorced, unemployed, and an alcoholic with too much time on her hands. Not wanting her roommate to know she lost her job Rachel takes the train into London each morning and home each evening. It so happens that the train passes the street where Rachel once lived with her then husband Tom. Tom now lives there with his new wife Anna - the lover who precipitated Tom and Rachel's divorce.
Rachel takes a great interest in her former street, and as the train passes each day she checks out the people living there and makes up stories about them. She's becomes especially interested in a couple whom she dubs "Jess and Jason". Rachel thinks the couple is blissfully happy until the day she spots Jess kissing another man in the front yard. Soon afterwards Jess disappears - and the mystery story takes off from there.
The couple's real names are Megan and Scott, and Scott soon becomes a suspect in Megan's disappearance. Rachel, knowing about the 'other man', feels compelled to insert herself into the investigation. She talks to the police and Scott, but because her alcoholism causes blackouts Rachel is deemed an unreliable witness. Rachel has other troubles as well. She's never gotten over her divorce and harrasses Tom and Anna with constant phone calls and the occasional unwanted visit.
The story is told from three rotating points of view - Rachel, Megan, and Anna - so the reader gets three perspectives on the events being narrated. To say much more would be a spoiler so I'll just say there are plenty of twists and surprises leading to a climax that's satisfyingly dramatic (if a little too long and drawn out).
The story is compelling and the characters are interesting but not particularly likable; these probably aren't people you'd want to hang out with. I enjoyed the book and would probably read more from this author.