Someone is killing women in Paris. Four have died in the past six
months, all of them young, with the same blood type, and drained of their
blood. The newspapers have dubbed the killer a "vampire." When it seems
that dancer Nora [Ronny Holiday] has become victim number 5, journalist Pierre Lantin [Dario MIchaelis] is on the story, much to the annoyance of police Inspector Chantal [Carlo D'Angelo], who
thinks of Pierre as a meddler looking for publicity.
Pierre learns that one of the murdered girls used to attend Ecole Jean d'Arc, so he heads over to the school to interview her classmates. All they remember (that they haven't already told the police) is that they were followed by a dazed-looking man in a trenchcoat just days before the murder. Pierre finds this interesting because the one picture he has of Nora also shows a strange man in a trenchcoat walking behind her. The next day, as Pierre drops off Lorette Roberts [Wandisa Guida], one of the students at the school, he notices the man in the trenchcoat. Pierre follows him to his apartment but, when Pierre returns with Inspector Chantal, he goes to the wrong apartment (they all look the same), and the Inspector is duly unimpressed.
Knowing that he's being tailed, Joseph Signoret [Paul Muller] (the man in the trenchcoat) goes straight to the Clinica Salus to see Professor Julien du Grand [Antoine Balpêtrè], for whom Joseph works in return for drugs. It's been Joseph's job to kidnap young girls for du Grand's experiments, but now he wants out. Du Grand's assistant gives him an out by strangling him. Du Grand, knowing that Joseph might have attracted the police, fakes his own death and goes into hiding on the advice of his cousin and benefactor, the duchess Margherita du Grand . Reduced to working in secret now, du Grand hitches up Joseph's body to a machine that circulates his blood and keeps his organs working, while the duchess Margherita demands that du Grand complete his experiments because she is giving a ball tomorrow evening and wants her niece Giselle [Gianna Maria Canale] to attend. Giselle, considered the most beautiful woman in Paris, has the hots for Pierre, but Pierre prefers to keep a wide berth around Giselle and her aunt Margherita, because Margherita destroyed Pierre's parents with her selfish love for Pierre's father. Still, Pierre is ordered by his editor to cover the ball for the society page, and Pierre reluctantly agrees.
Meanwhile, Lorette is stopped on the way home from school by a blind man asking her to deliver a letter to an address next to the Church of the Sacred Heart. When Lorette goes there, however, she is chloroformed and taken to the du Grand castle, where she is slated to become du Grand's next victim. When Lorette doesn't show up for dinner that evening, Pierre and her parents go to the police. The police find the blind man and get the address of the apartment, but the apartment has been abandoned.
The next evening, Pierre attends the du Grand ball. He fends off Giselle's attempts to interest him and leaves, but Pierre's partner Ronald Fontaine [Angelo Galassi], who is lusting after Giselle, remains after hours and scales the castle wall to get into her bedroom. When Giselle irately refuses his advances, her resulting anger shows in her face as she rapidly ages, turning into old Margherite. Now that Ronald knows Giselle/Margherite's secret, he cannot be allowed to live, so Margherite shoots Ronald and steps up her demand that Prof du Grand complete his treatment on her, even if it is not yet perfected. Reluctantly, du Grand prepares Lorette for the treatment, which involves hooking her and Margherite to the blood machine. After three hours on the machine, Margherite turns back into the beautiful Giselle and Lorette falls into a deathlike sleep.
The next day, Pierre runs into Giselle on her way into an art store. He watches as she writes a check using her left hand. This catches Pierre's attention because Lorette is also left-handed. When Pierre finds out that Ronald never returned from the ball last night, he puts 2 and 2 together and attempts to get Inspector Chantal to investigate. Chantal refuses to search the castle until Pierre provides with proof, so Pierre goes to the castle alone. There, he runs into Joseph, who has mysteriously awakened from his death and is lumbering around the castle grounds trying to get away. Pierre takes Joseph to the police, Joseph confesses his part in the murders, drops dead...again, and Inspector Chantal finally agrees to pay a visit to the castle.
The police search the castle but find nothing. Just as they are about to leave, Giselle reverts into Margherite before their eyes. This time a fevered search finds a passage they missed before. It leads to a hidden room where they are surprised to discover Professor du Grand alive and well just before they shoot him. Looking in du Grand's coffin, the police find Lorette, weak and in need of medical attention, but well enough to reveal her attraction to Pierre. While the ambulance takes Lorette to the hospital, Pierre and Chantal go out for coffee together. [Original synopsis by bj_kuehl]