Curse of the Fly is a sequel to the 1959 film Return of the Fly, although it does not yield its star, Vincent Price.
Motorist Martin Delambre attempts to keep evidence of his family's bizarre experiments in teleportation hidden from his wife, who is hiding secrets of her own.
Martin Delambre is driving to Montreal one night when he sees a young girl by the name of Patricia Stanley 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 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 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, 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