One late night at the DeLambre Frère Electronics Factory in Montreal, Gaston, the night watchman, hears strange noises emanating from the press room. After a bloodied woman runs from the press room and admits that she killed her husband Andre, one of the brothers who owns the factory, Gaston notifies François DeLambre of his brother's death. François then phones Inspector Charas, an old acquaintance, and asks him to go to the factory. When the jaws of the press are raised, they find Andre's body with its head and arm crushed, and François is baffled as to how Helene would know how to operate the complicated piece of machinery. After Dr. Ejoute, the family physician, declares that Helene is in a state of euphoric relief, Helene, calmly seated in her living room, admits that she killed her husband but refuses to explain why. As the inspector interrogates Helene, she is distracted by the buzzing of a fly. When the inspector asks to see Andre's laboratory, François is shocked that the equipment is a shambles. Believing that Helene may be insane, the doctor confines her to bed, and François assumes guardianship of her son Philippe. The inspector thinks that Helene is sane, however, and informs François that he intends to ask for an arrest warrant the following day. At dinner that night, Philippe mentions that his mother was desperately searching for a fly with a white head and strange leg. When François questions Helene about the fly, she, thinking that he possesses the insect, begs him for it. Instead he threatens to turn it over to the inspector unless she tells him the truth about his brother's death. After François promises Helene that he will kill the fly, she asks him to summon the inspector and then recalls a happier time, several months earlier when Andre excitedly showed her his new research project based on the disintegration and reintegration of atoms: After successfully transforming a plate, Andre decides to experiment on the family cat, but after being disintegrated in the chamber, the feline fails to reappear, and all that remains is a disembodied meow. Two weeks later, Andre reemerges from his lab, triumphant, and insists on transforming a guinea pig. After the guinea pig reappears, Helene makes Andre promise that he will not experiment on any more animals. One day soon after, Andre invites François to lunch to demonstrate his experiments, but when François and Helene approach the door to the laboratory, they find a note from Andre stating that he is unable to dine with them. Later, Philippe shows his mother a fly with a white head that he trapped, and she makes him release it. That night, Helene returns to the lab and Andre slips another note under the door, reporting that he has had a serious accident and is unable to speak, and asking her to bring him a bowl of milk laced with rum. Upon returning with the milk, Helene finds another note, instructing her to go into the other lab in search of a fly with a white head. He allows her into the room on the condition that she not look at him, and when she enters, she sees that he has draped a black cloth over his head. She then informs him that she made Philippe set the fly free, and when Andre reaches out in frustration, she sees a fly tentacle extending from his sleeve where his arm should be. Helene runs out of the room in horror, but promises to find the fly. The next morning, Andre hands Helene a note explaining that unknown to him, a fly entered the disintegration chamber with him and their atoms became entangled. If the fly cannot be found, Andre writes, he will be doomed to life as an insect and will kill himself. Helene immediately sends Philippe and her housekeeper Emma on a quest for the fly, but after they trap it in the living room, it slips through a crack in the window. When Andre learns this, he types Helene a note, insisting that all must be destroyed, including himself. Helene begs him to try the disintegration chamber one last time, and Andre humors her. After the process is completed, Helene pulls off the cloth and finds a giant fly's head staring at her. When she faints, Andre tenderly picks her up and caresses her face, but his claw begins to twitch uncontrollably. Sensing that his humanity will soon be overcome by the fly atoms, Andre smashes his equipment and then motions for Helene to follow him to the factory. There, he puts his head between the jaws of the press and gestures for her to close it, crushing his hideous head and claw. Helene completes her story, but the inspector, incredulous, refuses to believe her, and informs François that he intends to arrest her for murder. The next day, as François resignedly waits in the garden, a fly with a white head trapped in a spider's web squeaks "help me" at him, but he cannot hear the tiny voice. As the inspector and the attendants from the asylum arrive to take Helene away, Philippe tells François that he saw the fly in the garden. Running there, François and the inspector find Andre's head grafted to the body of the fly. The spider is about to devour the fly when the inspector crushes both insects with a rock. Now convinced of Helene's story, the inspector declares Andre's death a suicide. Some time later, François explains to Philippe that his father died in the search for truth.