The film follows Jane (Lewis) who has inherited a rent-controlled brownstone apartment from her late aunt, and, despite her boyfriend Greg's (Hurt) appeals, has decided to move in and live on her own for the first time. She meets Martha Stewart, (Duvall) her neighbor from the first floor, who tells her the names of all the occupants of the building, including the person who lives on the 4th floor, an old woman named Alice who is a hermit. The next morning, she finds a note on her door, warning her against the noise she was making while moving in. Jane quickly dismisses it and continues moving around her furniture. Another note appears and she calls a locksmith after someone tampered with her locks. The locksmith (Bell) is a strange man who lives in the building across from Jane, whom she suspected of harming a woman earlier.
Jane makes a friend out of Mr. Collins (Pendleton) a kindly old man who was also friends with Greg in high-school. After an argument with Greg about her refusal to move in with him, her tiles on her kitchen floor are smashed by her neighbor from downstairs and she calls the police. Furious, she gets her own back by making more noise. The next morning she falls down the stairs by tripping on a stair smeared with grease. She then tries to make amends by sending her neighbor a sorry note asking for a truce.
However things get worse as Jane's bathtub and apartment are infested with maggots and mice and she calls an exterminator (Costanzo) to handle the problem. She then finds out from a Korean grocer that Alice, the neighbor right beneath her, had suddenly stopped ordering her daily groceries. When she tries to tell Martha this, Martha turns against her, saying she has no respect for privacy or authority and that she's toeing the line by calling the police and talking to the Koreans. After the exterminator reveals that the mice infestation was no accident and that most likely her neighbor had been putting them through a drilled hole in her floor, Jane becomes terrified. She decides to break into the 4th floor to take pictures and gather evidence. There she finds mice and rats in cages, a typewriter and the ceiling completely mapped out with her furniture, along with word "Portcullis" written on a wall.
She then receives a call from the buzzer that a package is waiting for her. She opens the giant parcel to find it full of packing noodles, the same ones she found in the trash bags outside the neighbor's door, along with her sorry note saying "Truce accepted". She panics when she finds pictures of her aunt's dead body and decides to pack the evidence and go to the police. However, she is attacked and knocked out by the unknown neighbor and her evidence is stolen.
When she gets out of hospital, she tries to show Greg and the police the hole the neighbor drilled in her floor, but the hole has mysteriously disappeared. Greg then tries to convince Jane that no one believes her fears and that she should move out of the apartment and stay with him.
Jane goes back to the apartment and furiously bangs on the neighbor's door, yelling for him to come out. She is then comforted by Mr. Collins who takes her back to his place. She then notices strange patterns on Mr. Collins's ceiling and to her horror, sees the word "Portcullis". She is immediately knocked out by Mr. Collins, who drags her upstairs to the 4th floor.
Jane awakens in a room full of flies and she finds a dead body covered with maggots, presumably Alice. She tries to escape the apartment filled with packing noodles and hits Mr. Collins with a crowbar. She goes up to her apartment to call the police, but the line is dead. She then tries to escape through her front door, but her attacker has tied her door knob to the stairway railing. Mr. Collins breaks in through her window and Jane hides behind the fireplace shield. The locksmith appears and tries to fight off Mr. Collins, but he is eventually overwhelmed. Jane then stomps on the floor until a large decoration hanging over her doorway falls on Mr. Collins. She then runs out the door which had been untied by the locksmith, but is caught again by her attacker. Suddenly, Greg appears and asks Mr. Collins to hand over the knife he is holding against Jane. Mr. Collins then raises up the knife and Jane lunges at him, pushing him over the stair railing and down five flights of stairs to his death.
A few months later, Jane is seen in Greg's apartment talking to her friend Cheryl (Grdevich) from work on the phone, who tells her that the locksmith had something to show Jane. She replies that it is a good thing as she didn't get a chance to thank him. The final scene reveals that the locksmith has been looking through his window and sketching and painting what he sees. The camera pans through the room at all his paintings and the camera stops on a picture showing Greg and Mr. Collins talking at a table in front of the typewriter used to type threatening messages to Jane.Juliette Lewis as Jane Emelin
William Hurt as Greg Harisson
Shelley Duvall as Martha Stewart
Austin Pendleton as Albert Collins
Artie Lange as Jerry
Sabrina Grdevich as Cheryl
Robert Costanzo as The exterminator
Tobin Bell as The Locksmith
Lorna Millican as Mrs. Bryant
George Pottle as Mr. Bryant
Lara Brymer as uncredited role