Chapter 1: Rachel
* Rachel Watson is a divorced alcoholic who commutes daily past a suburban house she used to live in with her husband, Tom.
* She becomes obsessed with a couple she sees through the window, Megan and Scott, and creates fantasies about their lives.
Chapter 2: Anna
* Anna Boyd is the woman Rachel sees through the window. She is happily married to Tom and is pregnant with their first child.
* However, she is also hiding a secret from Scott: she had an affair with Rachel's ex-husband, Tom.
Chapter 3: Rachel
* Rachel continues to stalk Megan and Scott, believing they are a perfect couple.
* She confides in her therapist, Dr. Kamal Abdic, about her obsessions and her desire to reconcile with Tom.
Chapter 4: Anna
* Anna struggles with guilt and anxiety over her affair with Tom.
* She fears Scott will find out and leave her, especially with the baby on the way.
Chapter 5: Rachel
* Rachel witnesses a shocking event involving Megan and a man she doesn't recognize.
* She calls the police but is dismissed as unreliable due to her alcoholism.
Chapter 6: Anna
* Anna confides in her best friend, Lisa, about her affair.
* Lisa helps her rationalize her actions but also warns her of the potential consequences.
Chapter 7: Rachel
* Rachel becomes determined to find out the truth about what she saw.
* She teams up with her ex-husband, Tom, who is worried about Anna's safety.
Chapter 8: Anna
* Anna is kidnapped by a mysterious man who threatens her and blackmails her with the secret of her affair.
Chapter 9: Rachel
* Rachel and Tom investigate Anna's disappearance.
* They uncover a web of secrets and lies that surround Megan and Scott.
Chapter 10: Lisa
* Lisa becomes suspicious of her husband, Kamal, who is Dr. Abdic, Rachel's therapist.
* She suspects he is involved in Anna's disappearance and confronts him.
Chapter 11: Rachel
* Rachel confronts Megan about her suspicions.
* Megan reveals that she had a brief affair with Tom, and that she is now pregnant with his child.
Chapter 12: Anna
* Anna is rescued and returns home.
* She confronts Scott about her affair and they decide to work through their problems together.
Chapter 13: Rachel
* Rachel realizes the true extent of her obsessions and seeks professional help.
* She confronts her own trauma and the role it has played in her life.
Chapter 14: Epilogue
* Six months later, Rachel is in a better place, dealing with her alcoholism and rebuilding her life.
* Anna and Scott have a baby girl and are happily married. Megan and Tom continue to have a relationship.