Jill Johnson (Carol Kane) is babysitting the children of Dr. Mandrakis (Carmen Argenziano) at their home. When the children are asleep, Jill receives a telephone call from a man who asks her if she has checked the children. At first, Jill dismisses the telephone calls as a practical joke. However, as the calls become more frequent and threatening, Jill becomes frightened and decides to call the police, who promise to trace the caller if Jill keeps him on the telephone line long enough. Jill, frightened to extreme measures, arms herself as she receives one final call from the nefarious stalker. Soon after the conversation, Jill receives a call from the police, only to find out that the stalker is calling from inside the house. A light comes on at the top of the staircase, and Jill sees the stalker's shadow. In a panic, she immediately runs to the door, unhooks the chain lock only to reveal a close-up of a detective when she screams. The camera then zooms out.
Afterwards, Officer John Clifford (Charles Durning) investigates the matter. It turns out that the children were murdered by the perp several hours earlier. The killer is identified as an English merchant seaman named Curt Duncan (Tony Beckley), and is subsequently sent to an asylum.
Seven years later, Duncan escapes from the asylum, still psychopathic. Dr. Mandrakis hires Clifford, now a private investigator to find Duncan. Not knowing Clifford is after him, Duncan is now a homeless, vagrant loner. He gets into a fight and is beaten after disturbing a middle-aged woman, Tracy (Colleen Dewhurst), in a tavern, and later follows her to her apartment. Feeling sorry for his disastrous appearance and for the fact that his attempts at conversation with her started the fight in the first place, Tracy makes light conversation with him. While they are in the doorway talking, Tracy's phone rings. As she goes to answer it, Duncan lets himself and appears behind her when she gets off the phone. She explicitly rebuffs his awkward proposal to visit her for coffee the next night, assuming or hoping it will be the last of him she will see.
Meanwhile an increasingly obsessed and vindictive Clifford, having confided to a former partner (Ron O'Neal) that his intention is to kill Duncan rather than arrest him, follows the trail to the tavern where the fight took place, then to Tracy's residence the same night Duncan is likely to arrive for his visit. Clifford goes there and tells Tracy just how dangerous her situation has become, revealing that Duncan literally tore and hacked up Mandrakis' children with his bare hands, rendering them virtually unrecognizable. Tracy reluctantly accepts to be Clifford's bait at the tavern that evening, but Duncan does not arrive and she decides to return home. Clifford then leaves Tracy's place, but she is then attacked by Duncan, who was hiding in her closet. Tracy screams for help, and Clifford returns and chases Duncan away from the scene. In the streets of downtown Los Angeles, he loses Duncan's trail.
Jill Johnson is now an adult, married with two young children. One night, she and her husband Stephen go out to dinner in celebration of a promotion, and a friend named Sharon babysits her children. At the restaurant, Jill gets a telephone call from someone asking, "Have you checked the children?" Jill panics and calls Sharon. It appears that nothing is wrong at first, but then the call is suddenly disconnected. The police arrive and escort Jill back home and find that everything is fine. Clifford tries to call Jill, but finds that the line is disconnected. Jill and Stephen sleep. Later, Jill goes down for a glass of milk, when the lights suddenly go out. She goes back upstairs and returns to bed. She picks up the phone and discovers that the phone line is dead. The closet door opens a little, and she hears the voice of Curt Duncan. She tries to awaken Stephen, who turns around, revealing that it is actually Curt in the bed. He rips Jill's nightgown and chases her around the room. Clifford arrives and shoots Curt, killing him. Stephen is revealed to be in the closet, alive but seemingly unconscious. As Clifford comforts Jill, the last view is of the house, in view of the frightening eyes of Curt Duncan.
Output: What happens when the phone calls persist?