Amalia Olga Zubarry is a glamorous nightclub singer who, through a small barred window in her basement dressing room, witnesses a man dump a small body into a storm drain in the alley behind the club. The man, nicknamed the vampire serial killer by police, has been murdering little girls and disposing of their bodies without leaving a trace of his own identity. Government Prosecutor Roberto Escalada Bernard questions Amalia and she falsely denies having seen anything; if her occupation as a nightclub singer (a less-than-respectable job) is exposed, she could lose custody of her daughter.
In the meantime, a peculiarly shy, self-effacing language teacher, Professor Ulber Nathán Pinzón, who is always dressed in a black overcoat, is seen stalking a little girl. He disables the elevator in her apartment building and catches her on the stairs and kills her. The police set a trap for the killer by staking out a little girl on a sidewalk while they watch from a nearby car. The Professor, though tempted by this bait, narrowly evades it. By coincidence, however, the Professor is an admirer of Cora Nelly Panizza, one of Amalia’s co-workers at the nightclub, whom the Professor frequently visits, but only to stare at her, which aggravates his deviant lust for children’s blood.
Dr. Bernard Roberto Escalada is very solicitous of his wheelchair-bound wife, who exhibits the virtues of a saint. He is sexually frustrated, and when he questions Amalia again because he is sure she lied to him, he cannot resist the opportunity to kiss her, although Amalia rebuffs him. Amalia, however, admits to Dr. Bernard that she saw the vampire through her window and explains to him why she did not want to become involved. The police suspect that Gastón Pascual Pellicciotta, the nightclub owner, has returned to trafficking narcotics, and they stage a raid of his club in which he is shot and the club is closed. Out of a job, Amalia brings her daughter home from school and one afternoon leaves her with Cora while she departs on an errand.
It is at this point that Professor Ulber, the vampire, comes to visit Cora, who, not realizing that Ulber is the vampire, gets rid of him by telling him to take Amalia’s daughter out for a walk, during which Ulber takes her on rides at an amusement park, creating suspense that he may kill her too. Ulber has a habit of whistling a tune that he was heard whistling by some homeless street beggars while escorting one of the little girls he is going to kill. The tune, not identified in the film, is one of the numbers from the Norwegian composer Edvard Grieg’s Peer Gynt Suite, The Hall of the Mountain King. One of the street beggars, a Norwegian, Enrique Fava, recognizes it, and when he hears it again while Ulber is walking by with Amalia’s daughter, he sounds the alarm and all the beggars as well as the police chase and catch Ulber, rescuing Amalia’s daughter in the process. Unlike the dénouement in the film “M” (which inspired this film), in which the outcome of the serial killer’s trial is left untold, Ulber is sentenced to death, by hanging.
