Written by Harry Spalding and directed by Don Sharp, Curse of the Fly has been criticised for the fact that it does not match the continuity of the first two films. It does, however, build its narrative on elements and characters from those films.
In Curse of the Fly we meet Henri (Brian Donlevy) and Martin Delambre (George Baker), 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 Phillipe 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 Phillipe in Return of the Fly.
The dialogue within Curse of the Fly contains no mention of Phillipe, 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 Phillipe’s transformation in Return of the Fly.
As the film opens we meet Martin Delambre (Baker) who is driving to Montreal one night when he sees a young girl by the name of Patricia Stanley (Carole 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, and they 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 (Michael 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…