Martin Delambre (Baker) is driving to Montreal one night when he sees a young girl by the name of Patricia Stanley (Gray) running in her underwear. They fall in love and are soon married. However, they both hold secrets: she has recently escaped from a mental asylum; he and his father Henri (Donlevy) are engaged in radical experiments in teleportation, which have already had horrific consequences. Martin also suffers recessive fly genes which cause him to age rapidly and he needs a serum to keep him young.
In a rambling mansion in rural Quebec, Martin and Henri have successfully teleported people between there and London. However, previous failures resulted in horribly disfigured and insane victims who are locked in the stables. Martin's first wife is one of them, as are Samuels and Dale, two men who had worked as the Delambres' assistants. Martin's brother Albert (Graham) mans the London receiving station but wishes to terminate the teleportation project and escape the obsession that has driven his grandfather, his father and his brother.
The police and the headmistress of the asylum trace Patricia to the Delambre estate, where they learn that she has married Martin, but it is soon discovered that he had a previous wife whom he did not divorce. Inspector Charas, who had investigated Andre Delambre and is now an old man in the hospital, tells Inspector Ronet about the Delambre family and their experiments.
As the police begin to close in, a mixture of callousness and madness afflicts the Delambres, and they decide to abandon their work and eliminate the evidence of their failures. They subdue and teleport Samuels and Dale, but upon reintegration in London the two men are fused into a single writhing mass. Albert is horrified at the sight and kills the thing with an axe, destroying the teleportation equipment in the process. Tai and Wan (Burt Kwouk and Yvette Rees), a Chinese couple who had been helping the Delambres, have had enough and leave the Quebec estate.
Henri convinces Martin that they must send the unconscious Patricia to London and then follow in order to escape from the police. Martin resists, afraid that she might be harmed, so Henri volunteers to go first. Martin sends Henri to London, unaware that Albert has destroyed the reintegration equipment. Henri does not rematerialize and is lost. Realizing what has happened, Albert leaves the lab, sobbing, and is not seen again.
Inspector Ronet arrives at the estate, passing Tai and Wan as they drive away. Patricia awakens in the teleportation chamber but escapes before the transmission sequence is complete. Martin pursues her but starts aging again. Without his serum he quickly dies, sprawled across the front seat of his car. Soon after, Ronet finds him reduced to a skeleton, and he escorts the badly shaken Patricia back into the house.Brian Donlevy as Henri Delambre
George Baker as Martin Delambre
Carole Gray as Patricia Stanley
Burt Kwouk as Tai
Yvette Rees as Wan
Michael Graham as Albert Delambre
Mary Manson as Judith Delambre
Charles Carson as Inspector Charas
Jeremy Wilkins as Inspector Ronet
Rachel Kempson as Madame Fournier
Although a sequel to The Fly and Return of the Fly, the backstory used for Curse of the Fly does not match the continuity of the first two films. However, it does build its narrative on elements and characters from those films.
Curse of the Fly centers on Henri and Martin Delambre, identified as the son and grandson of the Andre Delambre character depicted in The Fly. Andre's invention of a teleportation device and subsequent accidental integration with a housefly remain within the backstory. However, his resultant assisted suicide is removed; instead, his son — apparently a different character from the boy Philippe Delambre depicted in The Fly — was able to put both the altered man and the altered fly back into the teleportation chamber and successfully reverse the integration, as was done with an adult Philippe in Return of the Fly. The dialogue within Curse of the Fly contains no mention of Philippe, although a photograph shown in the film, which is supposed to be of Andre in his altered form from The Fly, is actually a production still from Philippe's transformation in Return of the Fly.
The film was a box office disappointment.