In Los Angeles, California, Jill Johnson arrives to babysit at the Mandrakis residence. Before leaving for the evening, Mrs. Mandrakis informs Jill that she and her husband, Dr. Mandrakis, might see a movie after dinner, which means they could be gone until midnight. She also requests that Jill not disturb the children, who are asleep and recovering from a cold. During the evening, Jill remains downstairs and studies in the living room. After awhile, the telephone rings. When Jill answers, a man with a British accent asks, “Have you checked the children?” then hangs up. He calls back, repeating the question. As the same voice telephones several more times, Jill wonders if her friend, Bobby, is playing a practical joke. Rattled, she pours herself a glass of whiskey, then tries to reach Dr. and Mrs. Mandrakis at the restaurant, but they already left. Next, she contacts the police, but the officer says he is unable to offer much help unless the caller threatens her. When Jill answers another time, the anonymous caller poises the question more emphatically. Sensing that the man is outside watching her, she closes the curtains and bolts the front door. She does not pick up as the phone rings again, then redials the police. Sgt. Sacker reassures her that he will attempt to trace the call, and Jill manages to keep the stranger on the line for longer next time. As soon as he hangs up, the sergeant telephones and states that the call is coming from inside the house. He orders Jill to leave immediately. Looking up, she sees a man’s shadow at the top of the stairs. She frantically opens the front door as the police arrive. The intruder upstairs is apprehended, but he is covered in the blood of the two Mandrakis children, whom he killed several hours earlier. By the time detective John Clifford appears on the scene, the parents have returned home and are trying to comprehend the tragedy. Officer Charlie Garber informs Clifford that the killer, a British citizen named Curt Duncan, used a second telephone line inside the house to call the babysitter, who was not hurt. Seven years later, Clifford has retired from the police force and works as a private investigator. Dr. Mandrakis hires him to find Duncan, who has recently escaped from the state mental hospital. Clifford questions Duncan’s supervising physician, Dr. Monk, who believes the patient is no longer a threat to children and feels confident that the intensive therapy, including electric shock treatment, was helpful, but Clifford would prefer clues as to the whereabouts of the psychopath. Meanwhile, at a bar downtown called Torchy’s, Duncan tries to converse with a single, middle-aged woman named Tracy Fuller, but she ignores him. When Duncan remains persistent, another bar patron assaults him. Tracy suddenly feels sympathy for the British man as he is dragged into the alley. Walking home alone that night, she is unaware that Duncan follows her. He appears outside her apartment door and apologizes for his behavior at the bar. Tracy is initially considerate until Duncan insists on staying and having coffee. After finally persuading him to leave, she feels unnerved by the stranger. At police headquarters, Clifford receives cooperation from his former colleague, Garber, who is now a lieutenant, and combs through recent crime reports. He also questions vagrants on the street and shows them a photograph of Duncan. After inquiring about the fight at Torchy’s, Clifford tracks down Tracy and enlists her help in trapping the child murderer. Clifford then confides to Garber that he plans to kill Duncan and requests that the police do not get involved. Reluctantly, Garber agrees. As instructed, Tracy leaves Torchy’s and walks back to her apartment alone at night, while Clifford watches for anyone following her. Duncan, however, does not appear. Nevertheless, Clifford assures Tracy that he will keep patrol outside her building. As Tracy settles in for the evening, Duncan, who has been hiding inside the apartment, surprises her and pleads with her for friendship. Hearing Tracy scream, Clifford storms into the apartment, but is unable to catch the killer as he flees. With a tip from a vagrant, Clifford locates Duncan at a homeless shelter, and the two men confront each other. Clifford aims a lethal lock needle towards the murderer, but Duncan dodges the weapon and escapes. Meanwhile, Jill Johnson still lives in Los Angeles, but is now married to Stephen Lockhart and has two young children. As she prepares to cook dinner for the family, her husband telephones and announces that he is taking her to a restaurant. Jill then arranges for Sharon to babysit the children. While Jill and Stephen are enjoying dinner, the maître d’ at the restaurant interrupts and informs the couple that they have a telephone call. When Jill picks up the receiver, the familiar voice with the British accent says, “Have you checked the children?” Jill screams that Curt Duncan is back, and Stephen immediately calls the babysitter, who assures the parents that everything is fine at home. Accompanied by a police officer, Stephen and Jill return to the house, and are relieved to find that the children are okay. Nevertheless, Stephen places a rifle by his bedside. Meanwhile, at police headquarters, Garber hears about the incident with Mrs. Lockart and alerts Clifford. When Clifford telephones the Lockhart home and receives a constant busy signal, the operator notifies him that the line appears to have been recently disconnected. Meanwhile, Jill cannot sleep. Checking on her son, she notices a piece of candy in his room that she did not give him. Frightened, she goes back to her bedroom. There, she hears Duncan’s voice and assumes he is hiding in the closet. She shakes her husband in the bed, but instead, Duncan jumps out from underneath the covers. As the killer tries to strangle Jill, Clifford suddenly arrives and shoots Duncan dead. The private investigator comforts Jill and assures her that Stephen, who was knocked unconscious and placed in a closet, is fine.