In Montreal, Quebec, Canada, scientist André Delambre (David 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; and, thinking he knows the truth, Hélène explains the circumstances surrounding André's death.