Karl Brussard George Coulouris is a wealthy, vain New York City businessman who begins to behave oddly—answering telephones that aren’t ringing, forgetting that he owns ships in Miami, and failing to recognize his own physician, Dr Charot. After Charot shows him an x-ray of his head, Brussard admires it and says, "Ah, beautiful – the brain of Karl Brussard!" But Charot reveals he has an inoperable brain tumour and urges him to go to England, where Dr Phil R. Merritt Robert Hutton and his team are researching brain transplantation. Brussard, accompanied by his much younger companion Odette Vernet Nadja Regin, heads overseas.
In London, Brussard meets Merritt, his nurse Jean Cramer Julia Arnall, and Dr Lew Waldenhouse Sheldon Lawrence. Merritt confirms Charot's diagnosis, but Brussard is more fascinated by a living monkey's head in Merritt's laboratory. Merritt explains that its brain came from a monkey that had been dead for six years and that he and his staff have revitalised the brain and implanted it into the head. The brain, Merritt says, will “change its personality” to the one the monkey formerly had. Brussard decides he needs a new brain and, during a visit to Madame Tussaud's wax museum, learns of Nostradamus. He hires the drunken Dr Brandon Tony Quinn and they travel to France, open Nostradamus' crypt and steal his head. Brussard takes it to Merritt's lab. Without knowing whose head it is, Merritt's staff revitalise it. Brussard, who hasn't been told of this development, goes to the lab alone and finds the head, exclaiming, "It's alive! My brain! It's alive!"
> Ah, beautiful – the brain of Karl Brussard!
When Merritt tries to force him out of the lab, Brussard, in a rage, accuses Merritt of trying to kill him and damages the Omnigizer, a vital piece of medical equipment. After Jean repairs the Omnigizer, the head speaks, identifying itself as Michel de Notre Dame. Nostradamus! exclaims Merritt. He, Jean and Lew tell Nostradamus that his predictions have come true. "A great mind that can see into the future!" declares Brussard. "Worthy to be Karl Brussard!" When told that he's alive again, Nostradamus says, "It's against nature" and asks, "Why have you done this?" Brussard answers by yelling at Nostradamus, telling him that he (Nostradamus) is now him (Brussard).
Later, a mentally confused Brussard asks Nostradamus what he should do with his oil stocks. Nostradamus, knowing that stock prices are dropping, deliberately tells him to sell. Brussard does and is financially ruined.
Odette Vernet [Nadja Regin] has been secretly dating Lew Waldenhouse [Sheldon Lawrence]. Brussard discovers the affair, follows her to Lew's flat and strangles her. Lew arrives and finds Odette's body. Brussard, who has been hiding in another room, steps out with a revolver. Lew runs for his life, Brussard in pursuit. Lew heads for Merritt's lab and Brussard shoots him in the back, then flees. Merritt examines Lew and says that "his cranial nerves have been severed." He tells a police detective that Lew can't be saved. Brussard returns and shoots Nostradamus' head. Merritt decides to attach Nostradamus' head to Lew's body in an attempt to save them both.
Brussard returns again and discovers that Lew has become a monster, with Nostradamus' head encased in what appears to be a shoulder-width box covered with surgical tape. Brussard runs away; Nostradamus wanders off. Merritt calls Dr Alexander and tells him that Nostradamus "seems demented" and has "lost the power of speech." But then the police spot Nostradamus. Merritt and Jean race to a building with a bell tower and find Brussard chasing Nostradamus up a staircase. Brussard becomes dizzy and falls to his death. The bells begin to ring and Lew's body comes crashing down, leaving Nostradamus' head dangling in the bell ropes.
