In Montreal, the scientist André Delambre Vincent Price is found dead with his head and left arm crushed inside a hydraulic press, and his wife Hélène Patricia Owens confesses to the crime while refusing to offer a motive. Her behavior grows increasingly odd, driven by an unusual fixation on flies, including a supposedly white-headed fly. André's brother, François, pretends to know what happened but keeps key details to himself, prompting Hélène to urge François to bring the inspector in charge, Inspector Charas, so she can lay out the circumstances in a way that might expose the truth.
The story pivots to a gripping flashback that reveals a once-happy family dynamic: André, Hélène, and their young son Philippe Delambre Charles Herbert are shown living with a groundbreaking invention. André has been toying with a matter-transporter device—the disintegrator-integrator—first tested on inanimate objects, like a newspaper, to prove its feasibility. Encouraging results lead to bolder experiments involving living beings: a family cat that initially seems to vanish and reappear under the wrong conditions, followed by a guinea pig, all of which heighten the tension as the tests grow more ambitious.
With the tests approaching human scale, André constructs a man-sized pair of chambers and makes a startling confession. He explains, through typed notes and careful signaling, that a fly became trapped in the transport chamber along with him during an attempt to transfer himself. The outcome is extraordinary and terrifying: André’s head and left arm morph into those of a fly, while the insect acquires a miniature replica of his head and left arm—yet André still retains his human mind. This bizarre fusion creates a race against time as André realizes the fly may develop its own animal-driven instincts and threaten to overpower his humanity.
To reverse the process, André needs Helene to capture the transformed creature. The pair—and their house staff—spend endless hours scouring their home and the surrounding grounds for the elusive fly and its human-headed counterpart. The search leads Helene to a window crack where the fly escapes once more, and a creeping dread settles in as the fly’s supremacy grows, compelling André to act. Desperate, he guides Helene to the factory, and there he sets the hydraulic press in motion, signaling Helene to press the button and seal his fate. As the machine descends, André’s arm slips free, and in a harrowing moment, Helene tries to avert the catastrophe by lifting the press, replacing the arm, and restarting the apparatus in a doomed bid to salvage both lives and the truth.
Back in the present, Inspector Charas condemns Helene as insane and guilty of murder. Yet hope arrives in the form of Philippe, who reports a strange sight in the garden: a fly trapped in a spider’s web, carrying both André’s head and arm. François escorts Charas to witness the creature himself, and they behold the shocking sight: the fly with the fused human parts. A lurking spider approaches as Philippe’s details ring true, and Charas, in a final act of grim practicality, crushes both the fly and the spider to prevent any chance of belief in this extraordinary tragedy.
Faced with the impossibility of convincing the world, François and Charas resolve to declare André’s death a suicide, sparing Helene from a jury that would not accept the truth. The family’s fragile routine resumes: Helene, François, and Philippe carry on with their lives, the memory of the tragedy tempered by the quiet, stubborn pull of the truth they all shared in the shadows of the laboratory.
Sometime later, Philippe and Helene share a moment of croquet in the yard, while François arrives to take his nephew to the zoo. When Philippe asks about his father’s death, François answers with a somber reflection on the nature of discovery: 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 scene closes with Helene guiding her son and François away, leaving the house to its tense, unspoken memories and the unresolved questions that linger long after the credits roll.
