In Montreal, Phillipe Delambre (Brett Halsey) wants to vindicate his father by completing his experiments in matter transfer, but remembering the horrible consequences last time, his Uncle Francois (Vincent Price) refuses to help. So Phillipe hires Alan Hinds (David Frankham) from the family company, Delambre Frere, paying his salary himself. But the process is expensive and the money runs out. Francois gives in and funds the completion of the equipment. They use the transporter to store and re-materialize test animals.
Alan Hinds turns out to be an industrial spy named Ronald Holmes, wanted in England. He tries to sell the company’s secrets to someone named Max Barthold (Dan Seymour), but Holmes is confronted by a British agent. Holmes knocks him unconscious and stores him in the transporter, but when the agent is rematerialized, he has the paws of a guinea pig that was in the system, and a Guinea pig appears with human hands. Holmes puts the man and the guinea pig in his car and pushes it into the Saint Lawrence River.
Phillipe and Holmes fight over the irregularities and Phillipe is knocked cold. Holmes hides him in the same way, but with a touch of cruelty, he catches a fly and puts it in the transformer. Francois arrives and re-materializes Phillipe, but he comes out with a giant fly head and fly’s arm and leg. This creature runs out into the night, finds Max and kills him. He waits for Holmes to show up and kills him too. Then he returns home, where Inspector Beecham (John Sutton) has captured the fly with Phillipe’s head. They are placed in the transporter together and each comes out whole. We don’t learn Phillipe’s fate, nor anything about the thorny issue of whether he is responsible for the murders.
The film was directed by Edward Bernds as a sequel to The Fly. Vincent Price was the only star of the previous film to reprise his role. The movie’s budget was $275,000 and Price’s salary was $25,000. Price insisted on reading the script first and agreed to be in it based on a first draft, but re-writes cut out much of what he liked. The cane that Holmes wields closely resembles the wolf-head walking-stick from Universal’s The Wolf Man (1941). The film was designed to use the standing sets from The Fly. It received mixed reviews and was called an adequate sequel. But it has all the tropes—black and white atmosphere, spooky music, a machine with flashing lights and high-tech noises, a pretty girl threatened in her bed by a scream-inducing monster, and the revenge of the creature visited upon the bad guys. A nice touch is that Philippe has a phobia about flies.