In Montreal, Canada, 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 begins acting strangely. 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.