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In Montreal, Canada, at the funeral of his sister-in-law, Helene DeLambre, François DeLambre muses how Helene was destroyed by memories of a dreadful horror that had occurred years earlier. When an inquisitive journalist raises the specter of Helene's past, police Inspector Beecham, who investigated the death of André DeLambre, Helene's husband, silences the man. Upset by the journalist's queries, André's son Philippe asks François to retell the story of his father's death. Taking Philippe to the old family foundry, François shows his nephew André's ravaged laboratory and warns that his father's demise was a result of venturing into an area of knowledge in which man was not meant to go. François explains that André, who while researching the transmission of molecular structure in space through disintegration and reintegration of matter, was in the process of transmitting himself when a fly entered the chamber, transforming him into a man with the head and arm of a fly. With Helene's help, André destroyed himself by smashing his head and arm in a machine press. François' revelation only deepens Philippe's resolve to vindicate his father's experiments, and after hiring scientist Alan Hinds as his assistant, Philippe establishes a new lab at his late grandfather's mansion outside Montreal. After retrieving his father's papers, Philippe is distracted by the ominous buzzing of a fly. Three weeks later, Philippe is running out of money when François visits to voice his disapproval. When Philippe threatens to sell his half interest in the family factory to finance his research, François relents and agrees to back the project. Later, in Montreal, Alan visits Max Berthold, a well-known fence of stolen property who uses a mortuary as a front to conduct illicit business. Max, who knows that Alan is escaped British fugitive Ronald Holmes, agrees to sell the plans for the disintegrator-reintegrator, which Alan intends to steal. Inspector Evans, a British police officer on Alan's trail, follows to the mortuary, and later, Philippe drives past and spots Alan leaving the place. Sometime later, after successfully transmitting an ashtray and a guinea pig, Philippe decides to attempt a delayed reintegration, leaving a disintegrated rat in the chamber over night. That night, Alan sneaks back into the lab to photograph Philippe's diagrams. When Inspector Evans comes to arrest him, Alan throws him into the chamber and flicks the switch, causing him to disintegrate. When Alan reintegrates the inspector, he has the hands and face of the rat, and the inspector's head is now attached to the rat. Repulsed, Alan crushes the squealing rat, then drags the inspector's body to his car and drives it off a cliff. Upon returning to the lab, Alan is startled by Philippe, who inquires about the blood stain on the floor. When Philippe questions Alan's statements and produces the inspector's discarded handcuffs, the two struggle. Alan soon overpowers Philippe, knocks him unconscious and locks him in the chamber. Recalling Philippe's phobia of flies, Alan tosses an insect into the machine and then disintegrates them both. As Alan grabs the diagrams and runs out the back, François arrives, alerted by housekeeper Mme. Bonnard and her beautiful daughter Cecile. François tries to stop Alan, but Alan shoots him and drives off. After dressing François' wounds, Mme. Bonnard and Cecile help him down into the lab. Hearing police sirens wailing in the distance, François instructs Mme. Bonnard to delay the officers. Once in the lab, François throws the switch, and Philippe reappears, crowned with the head of a fly. While Sgt. Dubois pursues Philippe into the woods, a tiny fly bearing Philippe's head calls for help. After collapsing, François is transported to the hospital for treatment, and when he awakens, he demands to see Beecham. Beecham then arrives, and after François apprises him of the situation, Beecham volunteers to return to the lab and search for the fly. Philippe, meanwhile, stalks and kills Max at the mortuary while Beecham finds the squalling fly and places it safely in a jar. At the mortuary, Alan finds Max's body laid out on a slab, and Philippe then slips from the shadows, strangles him and stuffs his body into a casket. Soon after, François returns home to the news that Beecham has found the fly, but nevertheless worries that Philippe may no longer possess a human mind or consciousness. After Philippe appears, terrifying Cecile, Beecham leads him into the lab and places him in the chamber with the fly, and François then switches on the machine, miraculously restoring Philippe to his former self.