Attractive scientist Dr. Clare Wyatt (Anna Lee) fondly declines the proposal of journalist Dick Haslewood (John Loder), who loves her, and she goes to work for the reclusive Dr. Laurience (Boris Karloff), whom the scientific world regards as a crackpot. At Laurience's remote estate, she's greeted at the door by bitter, wheelchair-bound Clayton (Donald Calthrop), one of the doctor's more hopeless cases. Dick hangs around the neighborhood, sending stories to his newspaper, which is owned by his tycoon father, Lord Haslewood (Frank Cellier). Laurience demonstrates the reality of his discredited theory to an astonished Claire. Using an array of electrical equipment, he switches the minds of a good-natured and a cranky chimpanzee, then suggests to Claire that this could be done with people, making immortality possible. Lord Haslewood turns up unexpectedly, and offers the skeptical scientist the full use of the prestigious Haslewood Institute in exchange for the right to run stories on Laurience's progress, and to own the resulting patents. But problems arise when Haslewood sets up a conference of prominent scientists to hear Laurience explain the intent of his research. They all scoff loudly, and walk out. Lord Haslewood is furious, feeling he's been humiliated; he tells the shocked Laurience that he, not the scientist, owns everything, and orders him out of the Institute. Laurience suffers a mental breakdown, and invites Haslewood to his lab, where he straps him into the chair of his apparatus, then switches the minds of Haslewood and the crippled Clayton. When Haslewood realizes he's now in Clayton's body, he bursts into laughter and drops dead, puzzling the other two. Clayton is delighted to be in a body that's not only capable of walking, but is that of a financially powerful man. When Laurience offers Claire "eternal youth, eternal loveliness," she is shocked, and he realizes she's in love with Dick, so he makes plans for Dick to come to the Institute.
Meanwhile, Clayton learns why Haslewood laughed: Clayton is trapped in the body of a dying man: Haslewood had a weak heart. With his own scheme in mind, Laurience murders Clayton. He switches his mind with Dick's, giving the perfect alibi for murder -- and Laurience, in Dick's body, will have Claire. After the transfer, Laurience, in Dick's body, places Dick, in Laurience's body, in a chamber with poison gas. But when Claire arrives, she immediately realizes what has happened. Dick, in Laurience's body, awakes and falls out a window, gravely injured. On the street below, Sue encounters scientist Dr. Gratton (Cecil Parker), and convinces him to help her switch back the minds of Dick and Laurience before Laurience's body dies. Afterward, Laurience, in his own body, admits he was wrong, and begs Claire to destroy his equipment, then he dies.
A few years after this film, Karloff would make a series of movies for Columbia Pictures with plots very similar to this, but British film is far livelier than those; it's to the point, intelligent and entertaining. This was partly due to the clever script by L. Du Garde Peach, Sidney Gilliat and John L. Balderston. Gilliat later wrote several films for Alfred Hitchcock, and Balderston had co-written Dracula, among other notable films. Director Robert Stevenson was busy making a name for himself as a kind of back-up Hitchcock, though Hitchcock was not likely to have tackled The Man Who Changed His Mind, King Solomon's Mines or Non-Stop New York. Later, Stevenson came to the United States and gradually evolved away from thrillers to more romantic films, such as the 1944 Jane Eyre. His career was then spotty until he signed with Walt Disney, where he helmed many of the more prestigious films, including Mary Poppins, a far cry from the brisk horror of The Man Who Changed His Mind. Also known as The Man Who Lived Again, the U.S. title of this film is Brainsnatcher, and the U.S. reissue title is Dr. Maniac.