Jane, a Florida television newscaster, reports on the latest victim of a serial killer rapist, insisting that women be extra vigilant. That night, Debbie, a waitress, returns home from work and becomes frightened by threatening telephone calls from a stranger. Debbie telephones the police, but the officer believes she is overreacting to Jane’s report, although he promises to send an investigator the next morning. Later, Debbie notices her bedroom window open and grabs a kitchen cleaver as someone knocks at her door. When Debbie opens the door, a man wearing a mask scares her, but it is her boyfriend, Jeff, playing a joke. Responding to Debbie’s anxiety, Jeff consents to her wish to stay at his home and watches television while she packs. When the killer, Stanley Herbert, chops off Jeff’s head, Debbie finds her lover lying on the floor, but she assumes he is performing another ruse until she sees his head in her fish tank. Stanley then beats and rapes Debbie before strangling her to death with his belt. Meanwhile, Jane’s boyfriend, David, asks her to move into his apartment. However, Jane lives with her blind, deaf and mute sister, Tracy, and does not want to disrupt Tracy’s life. Jane returns to her double tower apartment complex and, as she parks in the garage, Stanley pulls into his parking spot nearby. Stanley is beltless and does not notice Jane watching as he changes out of his blood-stained shirt and puts it in a trash bin. The next morning, Tracy feeds her service dog, then makes breakfast for Jane. David unexpectedly joins them and is surprised when Tracy leaves to do laundry; Jane admits that Tracy handles many tasks on her own. Later, at work, Jane learns of the latest murders and reports another vehement warning on the air. While driving that afternoon, Jane stops behind a school bus and remembers a childhood incident, when she and Tracy were on their school bus that was followed by a child molester. When the girls got off the bus and reached a friend’s home, Jane left Tracy alone on the front porch and the child molester lured the girl into his car. Jane’s recollection is suddenly interrupted by a car horn and she continues home. When she arrives at the parking garage, she peers into Stanley’s car and checks the trash, but finds nothing. Jane phones the apartment building’s office, pretending that she hit Stanley’s car, and learns his name and apartment location, which is in the opposite tower, directly facing Jane’s apartment. That night, a secretary, Annette, works alone in an office building and receives threatening calls from Stanley, in which she hears music in the background. Scared, Annette telephones a friend who suggests she leave the building, but when she rushes out of the office, Stanley waits in her car and kills her. Elsewhere, Tracy swims in a pool while Jane discusses Stanley’s missing belt and stained shirt with David. He is a lawyer and claims the information is circumstantial evidence, but Jane is determined to investigate. Later, Stanley dumps Annette’s body into a gravel pit and his car becomes stuck in the mud. Stanley’s spinning wheels distract a couple having sex in a car nearby. When the man tries to help, Stanley stabs him in the neck, then slits his girl friend’s throat. Upon returning home later, Jane notices mud on Stanley’s wheels. That night, Jane has nightmares about Tracy’s childhood molestation which left her unable to see, speak or hear, although there was no medical reason for her condition. The next morning, Jane learns about the bodies found at a gravel pit and checks Stanley’s car, but it has been cleaned. She returns to her apartment and telephones Stanley, but there is no answer, and she sees his car leave the building. After jamming a vending machine, Jane goes to the maintenance office where a worker, Jimmy, offers to fix it. Left alone, Jane steals Stanley’s key from Jimmy’s cabinet. She lets herself into Stanley’s apartment, which is nondescript except for a cuckoo clock. Jane finds muddy shoes in the bedroom closet as Stanley returns. She stuffs one shoe inside her sweatshirt, sneaks onto Stanley’s porch and slips over the railing. When Stanley sees the open bedroom door and steps outside, Jane drops onto the porch below and escapes through the apartment of a surprised couple. Later, Jane reviews an interview she conducted with the friend Annette called the night of her murder and learns that Annette heard strange music over the phone. Jane wonders if Stanley’s cuckoo clock makes music. That night, Jane gives David the muddy shoe and asks him to have it compared to mud samples from Annette’s murder scene, but David thinks she has gone mad. Later, Jane retrieves a new gun from her purse, loads it with bullets and places it in her dresser drawer. She telephones Stanley and calls him a “phone freak.” When Stanley hangs up, she calls again, demanding that he turn himself in to the police. After a third call, Stanley is unnerved and he leaves the building in his car, driving until he sees a dancer entering a strip club. Stanley later follows the dancer home and kills her. The next morning, Stanley watches Jane’s television news report and, when she uses the term “phone freak,” he realizes that she is the person who called him. That afternoon, Stanley monitors Jane’s apartment from his porch and notices Tracy watering plants. When Jane returns home that night, she calls Stanley but there is no answer and she sees that his apartment is dark. Determined to check the cuckoo clock, Jane heads for his apartment. Meanwhile, Stanley parks his car outside, watches Jane go into his apartment, then heads for Jane’s residence. Inside, Tracy does not hear the telephone ring or David’s message that the mud sample from Stanley’s shoe matches the crime scene. Tracy claps for her service dog, but Stanley has already murdered it. Stanley moves items in the apartment, teasing the frightened girl and when he kisses her, it triggers a memory of her previous molestation. Tracy grabs a kitchen knife but Stanley wrestles it away. Although Tracy smashes a hot coffee pot on his head and hides in a closet, Stanley finds her and attacks. Tracy’s sight suddenly returns and she incapacitates Stanley with a kick between the legs. Meanwhile, in Stanley’s apartment, Jane discovers the cuckoo clock does indeed play music. As she looks out the window, she sees Stanley in her apartment and races for home. In Jane’s bedroom, Tracy finds her sister’s gun and shoots Stanley in the stomach. Tracy drops the weapon in the hallway and washes off the blood in the bathroom. Stanley, however, is not dead. He rushes into the bathroom and strangles Tracy, but Jane arrives, grabs the gun and kills Stanley with a bullet to his head. Jane hugs Tracy, who can now see, hear and speak.