Leo Kroll is a deeply troubled lab technician with an intense obsession for dolls, which he meticulously collects. Beneath this fragile exterior lies a dark and disturbing nature, as he is also a serial killer responsible for the deaths of several nurses. Throughout the film, Kroll finds himself under police interrogation regarding these murders, but he manages to convince the authorities of his innocence and is released each time.
Kroll's fascination with death and control extends to his personal life when he declares his intention to target Clara, a nurse caring for his ailing and possessive mother, who is hospitalized following a heart attack. Interestingly, as a chilling sign of his obsession, Kroll leaves behind a doll at the crime scene. This act hints at his psychological state, blending his fetish with his murderous impulses. Meanwhile, a subplot unfolds involving Kroll developing an infatuation with Tally, a girl working at an amusement park stall from whom he originally won the doll.
Despite passing a lie detector test and gaining suspect clearance, Kroll's tension escalates when he visits his mother and confesses to her the killing of Clara, which triggers a second, fatal heart attack. The story intensifies when Kroll visits the amusement park again and notices Barbara, Tally's coworker, talking to the police. His paranoia and nervousness become apparent as he tries to play a game at the park, missing ring after ring. When Barbara mimics a doll by saying "Mama," it triggers a violent outburst in Kroll, reminiscent of his troubled relationship with his mother. Overcome with rage and desperation, he goes to Barbara's apartment and murders her by strangulation as she steps out of the shower, a killing that disorients the police since it involves a different victim from his usual targets.
Following the death of his mother, Kroll experiences a fleeting sense of liberation, and it appears that his intense hatred for women begins to fade. He visits Tally and proposes marriage, but when she rejects him, his mental state deteriorates further. Kroll becomes convinced that the negative beliefs his mother ingrained in him about women are true. His obsession and psychosis push him toward more violent actions as the police narrow down their suspect through descriptions provided by Tally.
In a tense climax, Kroll hides in Tally's apartment, waiting to kill her when she returns home. The police, convinced that Tally might be the next victim, plant listening devices in her room and stay close to catch him. As Tally prepares to leave town, she accidentally damages the bug in her room, and unaware of the police listening in, Kroll emerges and begins to strangle her. The police storm into the apartment and open fire, hitting Kroll and causing him to fall from the window to his death. The fate of Tally remains uncertain, leaving viewers with a haunting impression of the devastating consequences of Kroll's disturbed mind.
