Bob, a lascivious middle-aged man, arrives in San Francisco and begins a string of serial killings, targeting young women. The first victim is Diane, Lisa Ramirez whom he meets at a bar. When she rejects him, he breaks into her house and shoots her and her boyfriend Barry, Dan Kosloff, to death. A short time later, a second victim’s body is found floating in the San Francisco Bay. Detective Michael Courtland, Warren Lincoln, begins investigating the case, which he suspects are related. He decides to postpone his impending marriage to his fiancée, Jennifer, Taylor Gilbert, by two weeks.
Jennifer is invited to stay with Michael’s wheelchair-using mother, Mrs. Courtland, Eve Brenner, at her spacious mansion outside the city. Jennifer is initially apprehensive but uses the opportunity to get to know Mrs. Courtland better. Unbeknownst to them, Bob has been casing the home. On Jennifer’s first night there, he breaks in and watches her sleep before entering Mrs. Courtland’s room. Mrs. Courtland awakens Jennifer, screaming, claiming someone was in her room. Jennifer dismisses this as a nightmare given that Mrs. Courtland had suffered a break-in some years prior. A paranoid Mrs. Courtland calls the police, but they find no evidence of a trespasser. It soon becomes clear to Jennifer that Mrs. Courtland is obsessive about protecting her home, compulsively checking that all doors and windows are locked.
In the morning, the housekeeper Helen, Najean Cherry, arrives on the property, and is stabbed to death by Bob moments after entering the house. While Jennifer takes the car to run errands, Mrs. Courtland is attacked by Bob, who threatens her with a pistol. Mrs. Courtland slashes his wrist with a butcher knife, causing him to drop the pistol in her lap. Armed with the gun, she barricades herself in the study and fires the gun at him through the door. She attempts to call police from another room, but Bob leaves the entryway phone off the hook, preventing her from doing so.
Jennifer returns at nightfall to find a disheveled Mrs. Courtland, and dismisses her claim of being attacked. Bob appears at the front door, and a surprised Jennifer goes to greet him—he is her father, William Witt. When she asks why he is there, he says he has arrived for her wedding, which she explains has been postponed. A shocked Mrs. Courtland claims Bob is the man who tried to attack her, but Bob assures Jennifer it was a mere misunderstanding, and that Mrs. Courtland wrongly assumed him an intruder. Jennifer offers her father dinner, during which he vaguely tells her he is in trouble. Meanwhile, Mrs. Courtland phones the police from upstairs, but they dismiss her claim.
Jennifer becomes suspicious of her father when she notices a kitchen window has been shattered, corroborating the story Mrs. Courtland told her. Bob lays out a series of photographs on the table of young women—each his victims—before revealing a forensic police sketch of him made by law enforcement. He pleads for Jennifer to help him evade authorities. He grows enraged when she attempts to call Michael, and violently drags Jennifer out of the house, but is stopped by Mrs. Courtland, who shoots him in the shoulder with a shotgun. Injured but still alive, Bob disables electricity to the house, and begins stalking Jennifer and Mrs. Courtland in the dark. Jennifer attempts to start the car in the garage, but finds it dismantled.
Returning inside, Jennifer finds her father with Mrs. Courtland, having overpowered her. While holding Jennifer at gunpoint, he is stabbed in the leg by Mrs. Courtland. Jennifer attempts to flee upstairs with Mrs. Courtland, pulling her up the staircase while fighting Bob off. Bogartis, a local officer summoned to the home by Michael, Stan Weston, who received a panicked call from his mother, is hacked to death by Bob outside with an axe. Cornered in an upstairs room, Mrs. Courtland pleads with Jennifer to help her kill Bob, who forces his way in with an axe. As more police arrive, Bob begins strangling Mrs. Courtland, despite Jennifer threatening to shoot him. Just before he is able to kill Mrs. Courtland, police enter and fatally shoot Bob.
