The story is a pretty transparent effort to repeat the success of The Curse of Frankenstein (1957), with Diffring engaged in dubious experiments in his attempts to cheat death, and having much the same kind of arguments with his (ostensibly) older mentor about their morality. There's quite a touch of Jekyll and Hyde to it too, which Hammer hadn't yet released an adaptation of, but given that they did in 1960 it must have been already in the works. Certainly, Dr. Bonnet turns very nasty, and indeed green, when he doesn't get his special potion.
This was the first of two Hammer films in which Lee played a character called Dr. Pierre Gerrard (the other being Taste of Fear), which must have made life easy for him. The character and his performance bear a lot of resemblance to his Paul Allen in The Two Faces of Dr. Jekyll, too - which is to say rather wooden and affrontedly bourgeois. lady_lugosi1313 will attest that I did not approve of his moustache - I rarely approve of facial hair on Lee, but this one was particularly bad, and as she noted, not even symmetrically stuck on. He appears to be fully on board with Diffring's immoral experiments, but in a twist reveal it turns out that he did not actually perform the operation he wanted. As he puts it, "I made the incision but did not perform the operation." This is probably for the best, because both of them seem to have a distinctly sketchy grasp of the thyroid gland's location.
Many other familiar faces were on board, including Roger Lloyd Pack, Hazel Court and Francis de Wolff. In the visual department, Jack Asher was hard at work on the cinematography, and everyone had exquisite period costumes. We also recognised the same doors with panels made of glass roundels as we had seen recently in The Snorkel and were used also in Revenge of Frankenstein, a blue chafing dish which gets around rather a lot in Hammer films of this era, Bonnet's fireplace, and the stairs down into his cellar, which had been reworked from both Curse of Frankenstein and Dracula. Presumably he was also using a lot of Frankenstein's science equipment and Dracula's books, too.
By the standards of Hammer's other classics in this era, it's a bit disappointing, being hide-bound in particular by an almost total absence of any exterior location footage. But everything ended with Diffring getting his comeuppance and the horrible legacy of his experiments being consumed in a flaming inferno, which is always satisfying.