In a secluded mansion on the outskirts of London, Dr. Laurience (Boris Karloff) creates a mysterious machine that can transfer the mind of one person into the body of another. His only companion is Clayton (Donald Calthrop), a man with a debilitating brain disease, kept alive by the doctor's experiments. Impressed by Laurience's work, Lord Haslewood (Frank Cellier), a wealthy publisher, funds his research at an Institute in London. However, when the doctor presents his theory to the Medical Society, he is met with skepticism and ridicule. Haslewood, now doubting Laurience's authenticity, withdraws his support.
Fueled by anger and desperation, Laurience uses his machine to swap the minds of Clayton and Haslewood, resulting in the deaths of both men. In his madness, Laurience plans to use the machine to achieve eternal youth by transferring his mind into younger bodies with his assistant Clare (Anna Lee). But when she chooses to marry Dick Haslewood (John Loder) instead, he decides to eliminate his rival and win her affections. In a final act of deception, Laurience transfers his mind into Haslewood's body, planning for the young man to be executed for a murder he committed. However, his plan is foiled and he is mortally wounded while fleeing from the police. Clare, in a last act of mercy, transfers their minds back into their original bodies. Dr. Laurience dies, taking his secrets and the machine with him.