Prostitute Ellen Tinker is brutally murdered in her New York apartment by a man who carries a pair of scissors in his pocket. Homicide detective Lt. "Mac" McCarthy later reveals to newspaper reporter Gardner that in addition to mutilating the victim, the killer cut her hair, which he then kept. Because McCarthy believes the murderer is insane, his partner Lt. Dana refers Gardner to psychiatrist Dr. Jason at Bellevue Hospital for more information.
When Gardner arrives, Jason is counseling a former patient, George Mastro, who was recently released from prison following his conviction for molestation. After Gardner consults with the doctor, Jason explains that compulsions such as the killer's involve a lack of self-control, which may indicate the killer is unable to determine right from wrong. George, meanwhile, boards a ferry and follows aspiring model Sue Grant to the upper deck. There he observes her accidental meeting with photographer Jan C. Verbig, who helps her collect some photos she has dropped. Sue is thrilled when Verbig hires her for an advertising job.
George just misses following Sue onto her bus after disembarking from the ferry. Sue later arranges to meet Verbig the next day, unaware that while they are talking on the phone, the photographer is shooting photos of burlesque star Lili De Mar, whom he loves. That night, model Mary Barrow is accosted in Central Park and murdered in the same manner as Ellen Tinker. Police question numerous men suspected of harassment. Gardner, whose editor wants him to write about the latest murder, sits in on an interview when police detectives question Joe Summers, a repeat offender who insists he is innocent.
Mac then visits Jason, who reviews his patient files for possible suspects. During Sue's first afternoon session with Verbig, she learns that Mary Barrow had worked for him six months earlier. Dana, meanwhile, finds Verbig's photo of Mary while searching her home for clues. The detectives question Verbig the next day, but he claims to remember little except that Mary worked as a burlesque dancer with Lili. Lili also fails to provide the police with further information. Verbig takes Lili out on a date that night, but when she learns he is not wealthy, she rejects him.
Verbig begins to hound Lili with flowers and telephone calls, but when he pays a desperate visit to her dressing room at the nightclub, she flatly refuses to see him again. One night, Verbig follows Lili home and murders her, then flees via the fire escape. A friend who had been at her apartment door finds Lili's body and rouses a janitor with her cries for help. Neither the janitor nor a patrolman capture Verbig, who later collapses at his studio. The next morning, Verbig's assistant Frank takes his suit to the cleaners. Mac and Dana question the janitor but he can give few physical details about the murderer's identity.
Suspecting that there may be residual hair in the murderer's pockets, Mac orders a citywide cleaners' search of men's suits. Mac later questions Lili's boss, Louis Quinto, about Lili's suitors, and Louis recalls Verbig's recent angry nightclub visit. Moments later, Det. Collins telephones Mac with news that he has found Verbig's suit at a cleaner's and that the pockets contained hair. Mac and Dana leave immediately for Verbig's studio, where he has become amorous with an unsuspecting Sue. Sue screams when Verbig becomes violent, and the police save her before Verbig can stab her.
Later at Bellevue, Jason injects Verbig with sodium pentethol to induce him to talk about himself. Jason learns that when Verbig was a child, he and his father found his mother kissing a man who was caressing her hair, after which Verbig's father committed suicide. Jason determines that Verbig is emotionally disturbed but knew that what he was doing was wrong. Verbig is later found guilty of murder and sentenced to execution. On the day of Verbig's death, a woman drops her newspaper on a streetcorner, and is joined by a stranger after he returns it to her.
