Child psychologist Anna Fox lives alone in Manhattan on West 121st Street after separating from her husband Edward; he lives away with their daughter Olivia, but she talks to them on a daily basis. Anna suffers from agoraphobia and her housebound state leads her to observe all of her neighbors from a second-story window, including the Russell family who recently moved in across the street. She also takes a large number of medications and drinks alcohol daily.
One evening, Jane Russell visits Anna and they befriend one another. She also meets Ethan, Jane’s teenage son, who suggests his father Alistair is abusive. One night, Anna witnesses Jane being stabbed to death in the living room. She contacts the police but they do not believe her, claiming everyone in the family is fine. Alistair arrives along with “Jane” who, to the shock of Anna, is a different woman from the one she met. She begins spying on the Russell family.
Anna’s tenant David, who lives in her basement, claims he did not hear or see anything, though she learns David was once in prison and broke his parole conditions. She receives an anonymous e-mail with a photo of her sleeping. She again contacts the detectives, who are joined by the Russells and David in Anna’s house, and Anna breaks down when it is revealed by a detective that Edward and Olivia are dead as a result of a car crash that Anna accidentally caused. Anna is now agoraphobic as a result and her medication has caused her to have hallucinations and conversations with people who are not there.
Anna apologizes to the Russell family and stops pursuing her suspicions. She records a video on her cell phone, planning to commit suicide by overdose. She then discovers a photograph she took of her cat and, in the reflection of a wine glass, sees the original Jane, proving she is real. Anna shows David the photo and he confesses the original Jane she met is a woman named Katie Melli, Ethan’s biological mother. Katie had been stalking the Russell family, trying to get close to Ethan. David refuses to help Anna prove the truth; he is then suddenly attacked and killed by Ethan, who had been lurking inside the house.
Ethan reveals to Anna that he murdered Katie and is a serial killer, having also killed Alistair’s secretary in Boston, and now intends to kill Anna. He had been letting himself into her house all week with a stolen key, and he was the one who took the photo of her sleeping. Anna flees to the roof, where they fight until she pushes Ethan through the skylight to his death.
As Anna recovers in the hospital, Detective Little visits and tells her they have arrested Alistair and Jane for helping Ethan cover up Katie’s murder and that they have found Katie’s body. Little admits he watched Anna’s suicide video, but hands back her phone to allow her to delete it before she has to return it for evidence. He apologizes for not believing her.
Nine months later, Anna, now sober and healthy, says goodbye to her house before she moves out and on with her life, now no longer afraid of the outside world.