At a fashionable Los Angeles jazz bar, handsome Stan Hess reads philosophy and is unresponsive when his latest girl friend reveals that she is moving out of town. Stan is upset, however, by the arrival of his elderly father, Will Belmont, with his new young wife Jayne. When Will chastises Stan for his rudeness to Jayne, Stan derides his father and mother for their numerous marriages. Later that evening, Stan goes to the apartment of Joyce Greenfield, knowing her musician husband is out of town.
Introducing himself as "Arthur Garrett," Stan tells Joyce that he has come in order to repay her husband a small loan. Impressed by Stan's polite manner and clean-cut look, Joyce allows him into the apartment. Moments later, Stan feigns a headache and requests water with which to take aspirin. When Joyce returns from the kitchen, Stan attacks and rapes her. Upon leaving the Greenfields', Stan steps in front of a car driven by police sergeant Dave Cullorah and, although unhurt, accepts a ride to the nearby hospital.
Before departing Stan takes note of Dave's address and the fact that he is married. At home, Dave is greeted by his wife Francee, who disregards his distrustful overprotective nature. Dave's partner, Jake Baron, then telephones to report Joyce's attack. At Joyce's home the next morning, Dave questions the veracity of Joyce's account, pointing out that there are the remains of two meals at her dining table. Furious, Joyce maintains the assault took place as she described. Back at police headquarters, Dave and Jake note that Joyce's attacker's behavior matches that of several assaults made by a man nicknamed the "Aspirin Kid," due to his method of using aspirin as a distraction during his crimes. Unsettled by Dave's blatant hostility toward Joyce, Jake advises him that because Dave's first wife was unfaithful, he should not mistrust the actions of all women.
A few days later, Dave and Francee meet Jake and his wife Marie for an afternoon at the beach. Francee confides to Marie that, to her delight, she believes she may be pregnant, but remains uncertain. Dave receives a call from a neighboring beach patrol informing him that a man matching the description of the "Aspirin Kid" has been sighted. Dave and Jake question Art Jester, whose confrontational responses result in his being taken in to headquarters for questioning. Unknown to the police, Art is an acquaintance of Stan, who runs a well-known beatnik beach hang-out. Dave summons Joyce to view a line-up that includes Art, but Joyce cannot make a positive identification. The questioning is disrupted by a call from Stan, who confesses and says that he wants to give himself up at a nightclub later that evening.
While Dave and Jake stake out the nightclub, Stan goes to Dave's house and, despite Francee's caution, gains entrance to the house. Using the same feigned headache technique, Stan attacks Francee, then leaves several cigarettes in the ashtray to suggest that Francee knew her assailant. The following morning, Dave is tense and distressed over Francee's assault yet unable to shake the suspicion that she might have prevented the attack. Although Stan telephones Dave at home to taunt him, several weeks go by with no further attacks and little progress in the investigation. One evening when Dave arrives home late from work, Francee confesses that she has learned that she is pregnant and fears the father may be the rapist. Dave forbids Francee to consider an abortion because it is illegal, but Francee vows not to have the child as long as any doubt remains about the father's identity.
Meanwhile, Stan blackmails Art into impersonating the "Aspirin Kid" in order to confuse the police. Art reluctantly agrees and following Stan's instructions, calls upon Georgia Altera, a striking divorcee who welcomes his visit. As Art is about to attack Georgia, however, her ex-husband Harry arrives, angering Georgia and disconcerting Art. When Harry insists that Art does not owe him any money, Art hurriedly departs. Suspicious, Harry insists that Georgia contact the police and Dave and Jake come by to question her. Georgia's attitude disturbs Dave, who wonders anew about a possible relationship between the attacker and victims.
At home that evening, Francee demands to know if Dave would accept the baby if they learned it was fathered by the rapist. Dave wavers, then claims that he would, but Francee continues to doubt him. The next day, Francee visits Marie to reveal her dilemma. Marie is dismayed over Francee's insistence that Dave does not want the baby and takes her friend to consult with neighbor Father Dinelli. Although not religious, Francee is struck by Father Dinelli's counsel and resolves to have the baby, then give it up for adoption. Dave begins following Georgia, certain that she is involved with Art. Disturbed by Dave's obsession with the investigation, Francee angrily tells him of her decision regarding the baby and her intention to move out. Soon after, Jake informs Dave that Francee is staying with him and Marie.
Undaunted by Jake's skepticism, Dave insists that he is certain the investigation hinges on the victims' involvement with the attacker. Dave then continues trailing Georgia over several months. One evening, Dave approaches Georgia in a bar and threatens to arrest her for harboring Art. Later, when Dave learns that Francee has gone into labor, he visits the hospital where Francee asserts that she has decided to keep the baby. When Dave hesitates, Francee asks him not to return until he can be honest about his feelings. Frightened by Dave's threat, Georgia contacts him to admit she has been seeing Art for several weeks and will be meeting him that night at the jazz bar. That evening when Art fails to arrive, Georgia telephones the number he has given her and reaches Stan, who tells Georgia to meet Art at the beach beatnik hang-out.
Georgia takes Dave to the beach location, but he is dismayed to find Art rather than Stan. Stan comes out of hiding to attack Dave, and takes him and Georgia hostage in the beach house. Infuriated by Georgia's fear, Stan is about to rape her, but is interrupted by his doped-up beatnik acquaintances. Georgia uses the opportunity to plead with Art, who then frees her and Dave. Dave pursues Stan out to the ocean, where he is wounded by a diving spear before overpowering Stan. Stan declares his hatred of women, then confesses to the attacks before breaking down. Stricken by Stan's vehemence, Dave later returns to the hospital. After Francee has given birth, she and Dave are reconciled and he embraces his new family.
