Description
In Montreal, Quebec, Canada, scientist André Delambre (Al Hedison) is found dead with his head and arm crushed in a hydraulic press. Although his wife Hélène (Patricia Owens) confesses to the crime, she refuses to provide a motive, and begins acting strangely. In particular, she is obsessed with flies, including a supposedly white-headed fly. André's brother, François (Vincent Price), lies and says he caught the white-headed fly; thinking he knows the truth, Hélène explains the circumstances surrounding André's death.
In flashback, André, Hélène, and their son Philippe (Charles Herbert) are a happy family. André has been working on a matter transporter device called the disintegrator-integrator. He initially tests it only on small inanimate objects, such as a newspaper, but he then proceeds to living creatures, including the family's pet cat (which fails to reintegrate, but can be heard meowing somewhere) and a guinea pig. After he is satisfied that these tests are succeeding, he builds a man-sized pair of chambers. One day, Hélène, worried because André has not come up from the basement lab for a couple of days, goes down to find André with a black cloth over his head and a strange deformity on his left hand. Communicating with typed notes only, André tells Hélène that he tried to transport himself but that a fly was caught in the chamber with him, which resulted in the mixing of their atoms. Now, he has the head and left arm of a fly; and the fly has his miniature head and left arm, though he keeps his mind.
André needs Hélène to capture the fly so he can reverse the process. Although she expends great effort in her search, she cannot find it and André's will begins to fade as the fly's instincts take over his brain. Time is running out, and while André can still think like a human, he smashes the equipment, burns his notes, and leads Hélène to the factory. When they arrive, he sets the hydraulic press, puts his head and arm under, and motions for Hélène to push the button. André's arm falls free as the press descends and, trying not to look, she raises the press, replaces the arm, and activates the machine a second time.
Upon hearing this confession, the chief detective on the case, Inspector Charas (Herbert Marshall), deems Hélène insane and guilty of murder. As they are about to haul her away, Philippe tells François he's seen the fly trapped in a web in the back garden. François convinces the inspector to come and see for himself. The two men see the fly, with both André's head and arm, trapped in the web as Phillippe told them. It screams "Help me! Help me!" as a large brown spider advances on it. Just as the spider is about to devour the creature, Charas crushes them both with a rock. Knowing that nobody would believe the truth, he and François decide to declare André's death a suicide so that Hélène is not convicted of murder. In the end, Hélène, François, and Philippe resume their daily lives. Sometime later, Philippe and Hélène are playing croquet in the yard. François arrives to take his nephew to the zoo. In reply to his nephew's query about his father's death, François tells Philippe, "He was searching for the truth. He almost found a great truth but for one instant, he was careless. The search for the truth is the most important work in the whole world and the most dangerous". The film closes with Hélène escorting her son and François out of the yard.
Industrialist François Delambre is called late at night by his sister-in-law, Helene Delambre, who tells him that she has just killed her husband, André. Reluctant at first, she eventually explains to the police that André invented a matter transportation apparatus and, while experimenting on himself, a fly entered the chamber during the matter transference.
