Someone is killing women in Paris. Four have died in the past sixmonths, all of them young, with the same blood type, and drained of theirblood. The newspapers have dubbed the killer a "vampire." When it seemsthat 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], whothinks of Pierre as a meddler looking for publicity.Pierre learns that one of the murdered girls used to attend EcoleJean 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 theywere followed by a dazed-looking man in a trenchcoat just days before themurder. Pierre finds this interesting because the one picture he has ofNora also shows a strange man in a trenchcoat walking behind her. The nextday, 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 withInspector Chantal, he goes to the wrong apartment (they all look thesame), and the Inspector is duly unimpressed.Knowing that he's being tailed, Joseph Signoret [Paul Muller] (the man in thetrenchcoat) goes straight to the Clinica Salus to see Professor Julien duGrand [Antoine Balpêtrè], for whom Joseph works in return for drugs. It's been Joseph's jobto kidnap young girls for du Grand's experiments, but now he wants out. DuGrand's assistant gives him an out by strangling him. Du Grand, knowingthat Joseph might have attracted the police, fakes his own death and goesinto hiding on the advice of his cousin and benefactor, the duchessMargherita du Grand . Reduced to working in secret now, du Grand hitches upJoseph's body to a machine that circulates his blood and keeps his organsworking, while the duchess Margherita demands that du Grand complete hisexperiments because she is giving a ball tomorrow evening and wants herniece Giselle [Gianna Maria Canale] to attend. Giselle, considered the most beautiful woman inParis, has the hots for Pierre, but Pierre prefers to keep a wide bertharound Giselle and her aunt Margherita, because Margherita destroyedPierre's parents with her selfish love for Pierre's father. Still, Pierreis ordered by his editor to cover the ball for the society page, andPierre reluctantly agrees.Meanwhile, Lorette is stopped on the way home from school by a blindman asking her to deliver a letter to an address next to the Church of theSacred Heart. When Lorette goes there, however, she is chloroformed andtaken to the du Grand castle, where she is slated to become du Grand'snext victim. When Lorette doesn't show up for dinner that evening, Pierreand her parents go to the police. The police find the blind man and get the address of theapartment, but the apartment has been abandoned.The next evening, Pierre attends the du Grand ball. He fends offGiselle's attempts to interest him and leaves, but Pierre's partner RonaldFontaine [Angelo Galassi], who is lusting after Giselle, remains after hours and scales thecastle wall to get into her bedroom. When Giselle irately refuses hisadvances, her resulting anger shows in her face as she rapidly ages,turning into old Margherite. Now that Ronald knows Giselle/Margherite'ssecret, he cannot be allowed to live, so Margherite shoots Ronald andsteps up her demand that Prof du Grand complete his treatment on her, evenif it is not yet perfected. Reluctantly, du Grand prepares Lorette for thetreatment, which involves hooking her and Margherite to the blood machine.After three hours on the machine, Margherite turns back into the beautifulGiselle 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 catchesPierre's attention because Lorette is also left-handed. When Pierre findsout that Ronald never returned from the ball last night, he puts 2 and 2together and attempts to get Inspector Chantal to investigate. Chantalrefuses to search the castle until Pierre provides with proof, so Pierregoes to the castle alone. There, he runs into Joseph, who has mysteriouslyawakened from his death and is lumbering around the castle grounds tryingto get away. Pierre takes Joseph to the police, Joseph confesses his partin the murders, drops dead...again, and Inspector Chantal finally agreesto pay a visit to the castle.The police search the castle but find nothing. Just as they are aboutto leave, Giselle reverts into Margherite before their eyes. This time afevered search finds a passage they missed before. It leads to a hiddenroom where they are surprised to discover Professor du Grand alive andwell just before they shoot him. Looking in du Grand's coffin, the policefind Lorette, weak and in need of medical attention, but well enough toreveal her attraction to Pierre. While the ambulance takes Lorette to thehospital, Pierre and Chantal go out for coffee together. [Original synopsis by bj_kuehl]
Output: Question: Julian experiments on Joseph's corpse to discover what secret?, Answer: How to make people young again using other people's blood