(Some Spoilers) Even though "The Curse of the Fly" is the third installment of "The Fly" "The Fly" doesn't at all appear in the film, except for a photo of the original "Fly". Henri Delambre, Brian Donlevy, and grandsons Martin & Albert, George Baker & Michael Graham, are in the movie continuing his fathers work in perfecting the tele-transporter to be used in the future for the benefit of mankind.
Even though Pop,Henri is as crazy and fanatical as can be in his pursuits to develop the tele-transporter his two boy's, Martin & Albert, are a bit turned off with his mad dreams that has taken a great toll on his health. Since it was making their lives a total wreck but still go alone with his research. Henri's experiments have taken a number of lives and horribly deformed some of the people that they were tried on, by putting them in that contraption, including Martin's wife Judith, Mary Manson and Martin and Henri themselves.
Martin driving one evening sees this young women running down the road dressed only in her bra and panties and takes her to the hotel that he's staying at in Montreal Canada. Learning that her name is Pat, Carole Stanly, Martin in no time at all fall in love with her and the two are married within a week.
Back at the Delmbra Mansion Martin continues his experiments with the tele-transporter with the help of his two live-in helpers the butler and maid Tie & Wan, Burt Kwouk & Yette Rees, with Pat blissfully ignorant of what happening in the house. When Martin's father Henri, back in London finds out that he did, got married he almost blows a fuse. Later finding out that Pat is an escapee from a local mental institution when Henri is tele-transported back to Montreal he realizes that now the police and hospital authorities will be snooping around the mansion. Henri gets Martin to show the police his marriage license with Pat to keep them off the premises,in order not for them to find out what he and Martin are doing there. There's another hitch in the plot when the police learn that Martin is still married to Judith, who mysteriously disappeared due to Martin's experiments on her, making his marriage to Pat illegal.
Even though both Martin and Pat agreed not to ask each others about their past Pat is suspicious about what Martin is doing behind her back. One night Pat find Judith playing the piano, Pat herself is a pianist, and is terrified at her deformed appearance. Later Pat to her horror finds a number of the persons that Martin used in his experiments locked up outside the mansion. Now Martin himself, his conscience got the best of him, comes clean with her telling Pat what he's been doing at the mansion all these years.
Not as good as the original "The Fly" but much better the "The Return of the Fly" with the emphasis on the tele-transporter and how in the end it destroys the Delambre's dream of perfecting it. Henri disappears when he's transported back to London with the receiving tele-transporter destroyed by his, also conscience stricken, son Albert. This has Henri turning into a bunch of loose and dismembered cells atoms and molecules floating in the nothingness of inner space forever.
