In Montreal, Quebec, scientist Andre Delambre (David Hedison) is found dead with his head and arm crushed in a hydraulic press. Although his wife Helene (Patricia Owens) confesses to the crime, she refuses to provide a motive and exhibits a number of strange behaviors. In particular, she is obsessed with flies, including a supposedly white-headed fly. Andre's brother, Francois (Vincent Price), lies and says he caught the white-headed fly; and, thinking he knows the truth, Helene explains the circumstances surrounding Andre's death.
In flashback, Andre, Helene, and their son Philippe (Charles Herbert) are a happy family. Andre has been working on a matter transporter device called the disintegrator-integrator. He initially tests it only on small inanimate objects, but he eventually 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, Helene, worried since Andre has not come up from the basement lab for a couple of days, goes down to find Andre with a black cloth over his head and a strange deformity on his left hand. Communicating with typed notes only, Andre tells Helene 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.
Andre needs Helene to capture the fly so he can reverse the process. Although she expends great effort in her search, she cannot find it and Andre's will begins to fade as the fly's instincts take over his brain. Time is running out, and while Andre can still think like a human, he smashes the equipment, burns his notes, and leads Helene to the factory. When they arrive, he sets the hydraulic press and motions for Helene to push the button. She activates the press twice - once to crush his head and once to crush his left arm.
The police, hearing this confession, deem Helene insane and guilty of murder. As they are about to haul her away, Andre's son Philippe tells Francois he's seen the fly trapped in a web in the back garden. Francois convinces the inspector to come and see for himself. The two men see the fly, trapped in the web, with both Andre's head and arm, looking terrified. It screams "Help me! Help me!" as a large brown spider advances on the creature. Just as the fly is about to be sucked dry by the spider, the inspector (Herbert Marshall) smashes them both with a rock. Thinking nobody would believe the truth, he and Francois decide to lie about the facts of the case so that Helene isn't convicted of murder. In the end, Helene, Francois and Philippe resume their daily lives, with Francois explaining to Philippe that Andre died doing the most dangerous act for humanity, but also the most beneficial: "the search for the truth".
What is caught in the chamber with Andre when he tried to transport himself?
A fly?