Martin Delambre is driving to Montreal one night when he sees a young woman by the name of Patricia Stanley running in her underwear.

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.

In a rambling mansion in rural Quebec, Martin and Henri have successfully teleported people between there and London, but the previous failures resulted in horribly disfigured and insane victims who are locked up in the stables.

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.

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 Dill, deliberately sending them together so that upon reintegration in London the two men are fused into a single writhing mass.

As they intended, Albert is horrified at the sight and kills the thing with an axe, but unknown to them, he destroys the teleportation equipment in the process.

Wan, angry at what they have done to Judith, sometimes releases her, ensuring that Pat sees her, but her husband and father in law insist she has had a nightmare.

Tai releases Wan and tells her what has happened, and they rid themselves of a possible murder charge by teleporting Judith's body.

The film was one of a series of movies Robert L. Lippert was making in England, in order to take advantage of the Eady Levy.

[1] Don Sharp later said he felt the script "wasn't good enough" and that he only took the job because he had been working for a long time as a second unit director on Those Magnificent Men in Their Flying Machines and was desperate to direct again.

[5] The Monthly Film Bulletin wrote: "Beginning with a stylishly eerie pre-credit sequence of Patricia's flight from the asylum projected in slow motion, The Curse of the Fly looks very promising for a time: the first glimpses of the Delambre laboratory and experiments, the uneasy courtship of Martin and Patricia, and their return to the quietly sinister Delambre mansion.