In New York City, child psychologist Dr. Anna Fox (Amy Adams) suffers from agoraphobia and lives alone with her cat Punch. She appears to still talk to her estranged husband Ed (Anthony Mackie) and their daughter Olivia (Mariah Bozeman). Anna is visited weekly by her psychiatrist, Dr. Karl Landy (Tracy Letts), who knows that Anna spies on her neighbors due to boredom, which she gets defensive about. She also takes several types of meds with wine, despite being advised against it.
Across the street, a new family, the Russells, have moved in. Teenage son Ethan (Fred Hechinger) comes over to give Anna a candle from his mother. He enters Anna’s home and talks to her, noting that he can see his dad Alistair (Gary Oldman) from her window. Ethan appears to tear up, and Anna says they can be friends. She lets him borrow some movies before he returns home.
Anna has a tenant, David Winter (Wyatt Russell), who lives downstairs in the basement. He occasionally offers to help with things, like the skylight that is molding and bound to break soon.
On Halloween night, some neighborhood kids pelt Anna’s house with eggs. While she tries to stop them, she is forced to open the front door, but cannot make it out before she passes out. She wakes up to find a woman from across the street (Julianne Moore) having helped her up. Anna assumes she is Jane Russell, Ethan’s mother. They sit and talk over wine, and Jane talks about how much she cares about Ethan. She also leaves Anna with a drawing that she made. Alistair later shows up and asks Anna if any of his family came around that evening, but she denies it.
The next day, Anna finds that David has been helping Alistair with some house work. Later that night, Anna hears a scream coming from the Russell home. She calls the police and then sees what looks like Alistair abusing Ethan. Ethan goes over to Anna’s house for comfort. Anna checks on David, who has a woman crying in his room, but he firmly denies that he heard a scream.
The whole following day, Anna appears to be suffering from delusions. She continues to watch the Russell home until she sees what looks like Jane being stabbed in the stomach. Anna attempts to leave the house to help, but she passes out again and is taken inside by Detectives Little (Brian Tyree Henry) and Norelli (Jeanine Serralles). Anna explains the situation, only to be met by Alistair and another woman who says she is Jane (Jennifer Jason Leigh). Ethan also shows up to deny that Anna has met his mother.
Anna starts to do some digging on the Russells. She learns that Alistair used to work for a firm in Boston, but then left after the mysterious death of his assistant Pamela Nazin. Anna calls the firm anonymously but then hangs up before her cover is blown. Anna then hears something from downstairs and goes to find David’s mail. He finds her and angrily confronts her about it, but then admits that he was in jail for a bar fight and is technically violating his parole since he’s supposed to be in Massachusetts.
Anna continues to watch the Russells, which Jane 2 takes notice of and threatens to call the police on Anna for. When Ethan comes over again, Alistair follows and strikes Ethan in front of Anna. He orders her not to continue meddling in his family’s lives.
Anna later receives an anonymous email with a picture of her sleeping. She believes David may have done it, but after bringing Little and Norelli in again, David says he was away with a woman, who is called and confirms to the detectives. The Russells show up for another confrontation, even after Anna presents the drawing that Jane 1 had left for her. It comes to a head when Anna mentions her family, and Norelli tells Anna that her family is dead, and that she has been hallucinating talking to Ed and Olivia, so she must have done the same in thinking something happened with the Russells.
A flashback shows Anna driving with Ed and Olivia on a snowy night. There is tension because Anna had an affair, and Ed wants to stop pretending like they are okay for Olivia’s sake. The phone rings, and the couple fights for it. Anna reaches to pick it up and takes her eyes off the road, leading her to veer over a hill and crash to the bottom, killing Ed and Olivia. Anna comes to terms with her delusions and apologizes to everyone.
Anna records a “last will and testament” video before planning to commit suicide by pouring pill powder into her wine. She then looks at a photo she took of Punch that shows Jane 1’s reflection in the wine glass. She shows the picture to David, who confirms that her name was Katie Melli and that she really was Ethan’s birth mother. She was a junkie who gave Ethan up at a young age and went to try and get him again even though Alistair tried to get Katie away from Ethan. Anna wants David to confirm this to detectives, but he refuses. David goes outside to answer the door, and a thud and grunt is heard. Ethan enters with a bloody knife, revealing himself as the killer. He admits to having been in her house through the week with her key, hence the photo she was sent. He also admits to killing Katie and Pamela, purely for the feeling. He wants to watch Anna commit suicide, but she tricks him and smashes a wine bottle over his head.
Ethan chases Anna outside. David tries to slow Ethan down and is stabbed to death. Ethan stalks Anna on the roof and manages to stab her in the face with a small rake. They fight until she pushes him through the moldy skylight, which breaks easily and sends Ethan falling to his death.
Anna wakes up in the hospital with Detective Little in her room. He says that Alistair and Jane are in custody, and Jane is telling everything, which proves Anna was right and the detectives are owing her an apology. They have also located Katie’s body. Little also says he saw Anna’s video, but gives her the phone to have time to delete it before it’s collected as evidence.
Nine months later, Anna moves out of her apartment with Punch and says goodbye to Ed and Olivia before finally moving on with her life.
