A phone-in caller named Bob tells deejay Jerry that when he moved to San Francisco, California, he was full of potential. Now, years later, he finds his potential has been squandered. He describes a night that went terribly wrong when he took a girl friend named Dianne to a bar, and she left with Barry, a man she just met. Bob followed Dianne and Barry to her apartment, where they made love. Although Jerry claims that Dianne has the right to choose the person she wants to be with, Bob insists that she went too far, but does not reveal the whole story: In a jealous rage, he killed Barry in a struggle with a gun, and then, Dianne. Later, as Bob watches television news at a grocery store, Detective Michael Courtland insists that Dianne and Barry’s killer will be caught. Later, Det. Courtland gets a call to examine a body found in the bay by a fisherman. As his fiancée, Jennifer, waits for him, she passes time at a nearby art gallery, and Bob follows her undetected. Afterward, Michael Courtland informs Jennifer he feels pressured to postpone their wedding until the murders are solved. Jennifer travels solo to visit Michael’s mother, Mrs. Courtland. Bob follows her there. Mrs. Courtland is confined to a wheelchair. Michael joins them for dinner, and shares gory details from his current murder case. He reveals that the female body found in the bay did not have a face. Soon, Michael is called back to work. In the middle of the night, Bob breaks a window, and finds Jennifer asleep. He brushes the hair from her forehead, then leaves without harming her. Minutes later, Mrs. Courtland cries for help, and claims that a strange man was in her room, but escaped out the window. Mrs. Courtland angrily insists that Jennifer check the cellar, and hands her a flashlight to carry out a search. On an intercom, Jennifer reports that the windows are locked and secure, and there is no sign of an intruder. Meanwhile, Bob listens from a hiding place. Mrs. Courtland telephones local police, and insists they send an officer to investigate. As Officer Tillman searches the property, he reveals that Mrs. Courtland often makes distress calls to his department. Once the officer leaves, Bob opens a basement window. In the morning, after the housekeeper, Helen, arrives for work, Bob stabs her in the throat before she can reach the intercom. Jennifer finds Mrs. Courtland in the cellar. When the older woman points out the open window, Jennifer claims to have checked everything, but Mrs. Courtland calls her a liar. Jennifer defends herself, then goes for a drive in Mrs. Courtland’s car. Bob watches Jennifer from a parked car across the street. Mrs. Courtland notices the front door open, and slowly makes her way down the staircase with a cane. Her wheelchair mysteriously rolls into the front door, and she hears the sound of breaking glass as she shuts the door. She wheels herself into the kitchen, sees broken dishes on the floor, and notices the patio door open. As she closes it, Bob breaks through the glass, and grabs her wrists. She bites him, and he releases her. As Bob backs her into a corner, Mrs. Courtland slashes his hand with a kitchen knife resting on the counter, and he drops the gun. She grabs the weapon, wheels into the study, and locks the door. She fires a bullet through the door, but misses. She is unable to call police because Bob has taken the telephone off the hook. When Jennifer returns, she sees the broken dishes. Mrs. Courtland explains the recent events, and shows Jennifer the gun. Jennifer takes the weapon to search the house. Wielding the kitchen knife, Mrs. Courtland checks the windows in the dining room. Jennifer calls Michael as she removes the bullets from the gun. She insists that Mrs. Courtland’s story is made up. The older woman demands to speak to her son, but Jennifer hangs up before she can talk. A knock at the door interrupts. When Jennifer answers, she sees her father, Bob, the man who has been following her. As they hug, he explains that he has come for her wedding, but she informs him that the event is still two weeks away. He responds that he just wanted to spend time alone with her before the big day. When they cannot find Mrs. Courtland, Bob asks for a drink. As Jennifer fixes his cocktail, Bob finds Mrs. Courtland, and she points the gun at him. He confiscates the firearm, as his daughter reveals that she removed the bullets. Although Mrs. Courtland insists Bob attacked her, he claims that he heard voices arguing earlier when he rang the doorbell. When no one answered, he walked to the back of the property, saw the broken dishes, and found Mrs. Courtland alone. When he tried to help her, she slashed the back of his hand. The older woman insists that Bob is lying, and curses him. As Bob persuades Jennifer to leave, Mrs. Courtland changes tactics. She apologizes for her excessive paranoia, and blames it on fatigue. She asks Jennifer to help her to bed as Bob walks into the living room, and loads a small pistol. Upstairs, Mrs. Courtland warns that Bob is very dangerous, and insists Jennifer remain by her side during the night. While Jennifer and her father talk in the kitchen, Mrs. Courtland calls police and requests protection. Meanwhile, Bob confesses that he is in trouble but refuses to explain. As they look at old family photographs, Jennifer notices that several windowpanes are broken. When she presses her father for more information, he shows her a police artist’s drawing, and admits he is the killer Michael is after. Bob asks for Jennifer’s help. She offers to fix him a drink, but calls Michael instead. Bob interrupts and hangs up the phone. As he drags his daughter outside, Mrs. Courtland appears in her wheelchair with a rifle, and turns on floodlights. He is blinded, and Mrs. Courtland orders Bob to release Jennifer. As Bob runs toward Mrs. Courtland, she shoots him in the shoulder. All three return to the house. Michael calls, and they each pick up the telephone in different rooms. As Michael advises Jennifer, Bob cuts off the electricity in the home. Jennifer and Mrs. Courtland decide their only chance is to escape. The older woman gives Jennifer the rifle, and orders her to get the car. Mrs. Courtland takes a kitchen knife, but as she wheels herself to the front door, Bob attacks. When Jennifer discovers that her father has damaged the car, she returns, and sees Helen the housekeeper dead in the wheelchair. Bob holds Mrs. Courtland hostage, and orders Jennifer to drop the gun, and pray. Jennifer distracts her father, allowing Mrs. Courtland to stab him in the thigh with the kitchen knife. The women evade Bob and climb the stairs, then lock themselves in the study. They watch from the window as Officer Tillman approaches. Bob drives an axe into Tillman's back. Mrs. Courtland wants to kill Bob, but Jennifer refuses to murder her own father. Bob uses the axe to break down the study door, and approaches the corner where the women hide. Mrs. Courtland cocks the hammer of the gun, but Jennifer grabs it. In the process, the gun goes off and the bullet hits Bob. As he heads for the door, he describes how Jennifer will visit when they lock him in prison. Jennifer hugs her father, and he smiles. Soon, they hear the sound of police sirens, and Bob insists the women plotted his capture. As he chokes Mrs. Courtland with the handle of an axe, Jennifer points the gun at her father, and pleads with him to stop. Michael arrives with two officers, and shoots Bob dead. Michael comforts Jennifer, and an officer tells Mrs. Courtland her ordeal is over. Michael searches Bob’s wallet, and finds a touching snapshot of Bob with his arm wrapped around his six-year-old daughter.