A person is driving around Los Angeles at night, and a flashback shows a car wreck, an ambulance, and a little girl lying dead on a gurney. The car stops and a person grabs a toolbox from the trunk and heads to a two-story apartment building. The unseen person walks into an apartment, and the resident, Mrs. Andrews, walks out of the bathroom, letting the man know that she called a couple of day ago. The unseen man attaches a drill bit and heads towards the woman. The woman breaks a bottle and holds the fractured glass against him, but the man grabs her arm and drills into it. She runs to the bathroom and locks the door, but the man drills through the door, gets inside, and punches Mrs. Andrews. He carries her to the main room where he drills her to death while a record plays. He puts the bloody drill back in his toolbox, puts on a ski mask, and leaves the apartment.
A little later, Debbie (Marciee Drake) returns to her apartment and changes clothes. She opens a door to get something in the hall closet and the ski-masked killer punches her out and carries her to the stop of the building complex stairs where he beats her to death with a hammer. He takes the body back into the apartment and places it on the floor. Maria (Evlyn Guerrero) another tenant, arrives at the front door to ask about the mess outside. When she gets inside, the killer keeps her from leaving, and stabs Maria to death with a screwdriver. In another apartment, the ski-masked killer sees a woman with a man, and avoids the room. An elderly woman sees blood at the top of the stairs and calls out to a man, and they both knock on Debbie's front door. The man opens the door and sees the two dead women.
A little later, the police are on the scene and they speak with Van Kingsley (Cameron Mitchell), the owner of the building. They ask for a list of tenants, and he tells them that he screens all of the applicants. The next morning, teenager Laura (Pamelyn Ferdin) talks with her brother Joey (Nicholas Beauvy) and leaves for school with one of her friends.
That night another one of the tenants, Dee Ann (Marianne Walter) is getting ready to take a bath and the humming ski-masked killer is outside with his tool box. He cuts the chain lock on the front door and walks into the bathroom where he sees the woman in the tub. Dee Ann sees the killer and jumps out of the tub and throws a towel at him. He shoots a nail gun at her but misses. He reloads and misses again. In the main room, Dee Ann tries to persuade the killer to put the nail gun down, but after a quick flash of the girl and the accident, the killer shoots and hits Dee Ann in her stomach. She leads against a wall and he fires the nail gun into her head.
In Laurie's apartment, she is on the phone with her boyfriend when the killer walks in just as she is hanging up. He grabs her from behind and she struggles until she passes out. A little while later, Joey returns to an empty apartment and he gets a drink for his alcoholic mother when she comes home. They hear screams and run outside where the tenants have crowded around Dee Ann's apartment. The police led by Detective Jamison (Tim Donnelly) arrive where they talk to Joey and ask him where he was the last few nights. Joey, anxious to find his sister, heads to Mrs. Andrews' place where he sees Kent (Wesley Eure) who is there to clean up the bloody mess. The two discuss that wreck which resulted in the death of Kent's cousin Cathy. Joey says that he wants to know here Laurie is, and the two head over to Dee Ann's apartment. After looking around, Kent sees a bloody nail in the wall, and he surmises that the killer used a nail gun to kill Dee Ann. The two boys leave.
Joanne (Aneta Corsaut), Laurie and Jody's mother, heads to her place of employment, a local bar, because she does not want to be home to think about the kidnapped Laurie. Joey and Kent drive to his uncle's house for cleaning equipment. In the garage, the see Vance who simply hands Kent some money and shuts the back door. The boys get some supplies and leave. Vance watches them leave and goes to the bedroom, where Laurie is bound and gagged on the bed. He brushes her hair and talks to her about his daughter Cathy and the evil world. He says that he killed all those women at the apartment building because they were evil. Laurie calls him 'Daddy' and asks to untie her. Vance says that he cannot lose her again and he leaves Laurie to cry alone.
Meanwhile, Jamison goes to the bar to talk to Joanne about Laurie's disappearance, but does not seem to get any useful information from her. Kent goes to his uncle's place and sees Laurie there in bed and gently raps on the window. The next day, Joey goes to Mrs. Andrews' apartment, where Kent is cleaning the place. Jamison is looking out the window with binoculars. Jamison tells Joey that the killer forced his way into one apartment, but may have conned his way into the others. Joey asks if the killer had to con his way at all, and Kent quickly eyes the two. Joey leaves, gets his mother's car keys, and heads over to Vance's place. Inside the garage, he finds the toolbox and a bloody drill bit. Kent enters and tells Joey that he is crazy for believing that his uncle is a killer. Kent says that if there is anyone who can believe Joey's claims, the would be in so much trouble with his uncle. Kent throws some gasoline on Joey and tells him that he must protect his family. He lights some matches and throws one on Joey who burns to death.
Kent walks into the bedroom where Vance is with Laurie, and he tells his uncle that he is sick and insane for keeping Laurie there and killing the other women at the apartment complex. Kent further agitates his uncle by saying that he and Cathy had sex on several occasions. Vance angrily chases Kent out of the bedroom and into the kitchen where Kent arms himself with a knife. Vance grabs the knife and the doll that he is holding loses its head. The deranged Vance screams: "you killed my Cathy!" Vance lunghes at Kent and onto the knife and falls to the floor. Kent picks up the doll, puts its head back on and cleans the blood off while Vance lies still where he fell.
Kent returns to the bedroom and cuts Laurie's restraints with scissors. Laurie is overjoyed and thanks Kent and persuades him to call the police. Kent tries to kiss Laurie, but she turns away. Kent suddenly slaps her and climbs on top of her ignoring her screams as he presumably rapes her.
A little while later at nightfall, Kent and Laurie are lying side by side in bed and Kent is talking about the possibly of marriage. Laurie asks about her brother and Kent avoids the question by saying that they will be together. Laurie apparently figures out that Kent may have killed Joey. Laurie turns and eyes the pair of scissors on the night stand next to her and grabs them.
In the final shot, a benumbed Laurie, still holding the scissors and with blood splattered all over her white dress, slowly walks through a deserted parking lot of a local shopping center as she heads for home as dawn starts to break in the distance.