Anna Fox lives alone in an upscale Harlem neighborhood. She is separated from her husband, who has custody of their 8-year-old daughter. Anna suffers from agoraphobia, a condition that makes her unable to leave the house. To fill time, Anna watches old black-and-white movies. She also takes copious amounts of prescribed medications and drinks wine. A licensed psychologist, she dispenses advice to others with agoraphobia on a website called The Agora.
Anna likes to watch her neighbors. She views their lives through the zoom lens on her camera, documenting their normal days as well as their arguments and affairs. She is intrigued when a new family, the Russells, move in across the street from her. For several days, she sees only the movers, the father and his teenage son. There is no sign of a Mrs. Russell.
The son, Ethan, comes over one afternoon to give Anna a present from his mother. Having been a child psychologist, Anna finds herself slipping into her doctor persona in order to draw the shy Ethan out and learn more about him. He is 16, home-schooled and misses his friends back in Boston.
The following night, Halloween, pranksters throw raw eggs at Anna’s door and windows. She summons her courage and steps outside to frighten them away, but succumbs to a panic attack. A woman helps her back into her home. When the stranger says she was on her way next door, Anna assumes she is Jane Russell, Ethan’s mother. They talk about Ethan and Jane shares a photograph of the boy when he was only 5.
A few days later, Jane visits again. The two women share several drinks and talk about their lives. Anna admits that she has been housebound for 11 months. Jane sketches a picture of Anna and signs it. Later that evening, Jane’s husband, Alistair, comes over and questions Anna. He asks if she has had any visitors that day. Having been warned by Jane that he is controlling, Anna lies and says she has been alone all night. He seems unsettled when Anna tells him to thank his wife for the candle Ethan brought over several days ago.
Besides the Russells, Anna has regular conversations with her husband, Ed, and their daughter, Olivia. Although Ed worries about Anna, and their conversations are friendly, no mention is made of them trying to get back together. Anna also has visits from her psychiatrist and her physical therapist, Bina, whom she now considers a friend. Both are helping her recover after an accident that occurred 11 months ago. Anna also speaks to her tenant, David, a young man who lives in the basement of her house.
One evening, Anna hears a scream coming from the Russells’ house. When she calls, Ethan nervously tells her that no one is hurt. Anna calls again, but this time, she is rebuffed. Alistair assures her that no one screamed. When she hangs up, Anna sees Jane leaving, unharmed. Ethan comes over a short time later saying that he needed to get out of the house. Before he goes back home, Anna gives him her cellphone number and tells him to call anytime he needs help.
Anna finds herself opening up to an online friend, Lizzie. She begins telling Lizzie about a trip she, Ed and Olivia took the previous winter. She and Ed had already decided to separate, but they had not yet told Olivia. Instead of canceling the trip, they decided they would go to the lodge in New England together and then tell their daughter about their decision.
When they arrived, Ed could not keep up the charade. Olivia had been furious and insisted they go home. Anna pauses while typing the story into the computer and discovers Lizzie has signed off. Afraid that she has somehow offended Lizzie, Anna begins drinking.
Anna wakes up later that night on her couch. When she looks out the window, she sees her friend Jane arguing with someone in the house. Still intoxicated, Anna finds her camera so she can zoom in closer to see what is happening. When she looks again, Jane staggers toward the window with a knife hilt sticking from her chest.
Jane looks right at Anna and reaches toward the glass. She slips from view, leaving a smear of blood on the window. Anna finds her phone and calls 911 to report a murder. After giving the police her address, Anna drops the phone and goes outside to try and help her friend. Before she can reach the Russell’s house, she collapses in another panic attack.
Anna wakes up in the hospital, and Detective Little questions her. Anna tries to convince him that Jane has been stabbed. Little takes her home, heavily sedated. He replays her 911 call so she can hear how slurred and almost incoherent she sounded. His partner brings Alistair over to Anna’s house to assure her that his wife is very much alive.
Alistair goes to his house and returns a few minutes later with Ethan and a stranger he swears is his wife, Jane. Anna feels as if the world has turned upside down, especially when Ethan does not refute his father’s claim. After the Russells leave, Detective Little gives her his card and tells her to call him if she needs anything in the future.
Convinced she saw her friend murdered, Anna watches the Russells’ house. The new Jane sees her spying. She calls to tell her to stop or she will tell the police. Anna questions her tenant, David, asking if he ever met Jane, but he insists he didn’t. He also didn’t hear a scream earlier in the week.
Anna retreats to her room and exchanges messages with her friend Lizzie. Afterward, Anna recalls the trip to New England with Olivia and Ed. At Olivia’s insistence, the family started to drive home, but a blizzard soon made driving difficult. When Anna’s cellphone rang, Ed grabbed it, threatening to speak to Anna’s lover. As she tried to take the phone back, the car went off the side of the road and into a gorge. Anna survived the crash and spent 36 hours trying to keep Ed and Olivia alive. They were all trapped on a narrow ledge. David interrupts her thoughts, and the two share several glasses of wine. They end up sleeping together.
When Anna summons her courage to follow the new Jane to a local coffee shop and confront her, Jane calls the police. Detective Little warns Anna to refrain from stalking the family. Later, when she tries to get into her cellphone, she is told her password has been changed. She has to go into her computer to unlock it again.
The following day, Anna receives an email from “guesswhoanna” with the subject “Jane Russell.” It is a picture of Anna sleeping in her bed. The time stamp indicates it was taken at 2 a.m. that morning. Detective Little returns to investigate, but shows her that the photo was taken with her phone and so is probably a selfie. He has spoken to her psychiatrist and discovered that Anna is not separated from Ed. Ed and Olivia died in the accident. The police believe she has concocted lies to garner attention.
Anna self-medicates over the next few days and becomes suicidal. She forces herself not to talk to Ed and Olivia. She initially wonders whether she is going insane but then wonders if the woman she called Jane existed, but only pretended to be Jane Russell. Alistair breaks into her house one night and returns a key that she had given Ethan. Alistair chokes Anna while warning her to stay away from his family.
The following day, Anna deletes the photo of herself sleeping from her phone. The previous picture comes on the screen. It is one she took of a sunset through the window the night her friend Jane visited. Anna is shocked to see that Jane is reflected in the window. Anna calls and asks Ethan to come over.
When he arrives, she shows him the picture. He finally admits that the woman was his biological mother, Katie. She had tried to re-establish contact with him but Alistair refused her presence in their home. Katie kept coming back, eventually talking to the real Jane Russell, but it was Jane who killed Katie during an argument. Ethan begs for the chance to talk to his parents before Anna calls the police to tell them everything.
Anna waits in her bedroom, drifting off to sleep, when she gets a text from Ethan saying they are going to the police. She wakes up later to find Ethan in her room. He never told his parents anything because he actually killed Katie. His parents moved to New York because he had gotten in trouble harassing an older woman in Boston.
A psychopath, he has been stalking Anna online (pretending to be Lizzie) and has been in her house at night. Now he intends to kill her but make it look like she committed suicide. Anna manages to kick him away and escape to her roof garden. He chases her there, and she pushes him through the skylight. He falls to his death.
Saving her and proving to others that she was not insane gives Anna the strength to begin healing. She gives up alcohol and takes the proper doses of her medications. Six weeks after Ethan’s death, she steps out of her house with her friend Bina by her side.