Dr. Laurience Boris Karloff, once a respected scientist, isolates himself in a secluded manor and begins to probe the origins of the mind and soul. He is aided by the capable surgeon Dr. Clare Wyatt Anna Lee and a wheelchair-bound confidant named Clayton Donald Calthrop, weaving an atmosphere of meticulous, almost obsessive work. The scientific community rejects his theories, threatening everything he has labored to build, and Laurience presses on, his work framed in whispers about genius and danger, while his own sense of identity tightens around the unusual pronunciation of his name, Lorenz.
To safeguard his research, Laurience turns to brain transference as a vehicle for his own nefarious aims, swapping the mind of generous philanthropist Lord Haslewood Frank Cellier with the volatile personality of Clayton. With Haslewood's wealth and prestige at his command, the scientist grows bolder, an almost unstoppable force in the pursuit of power rather than truth. The body in charge becomes a tool for conquest, and Laurience revels in the control he wields over fortune and influence, even as he keeps the operation's ethical vacuum well hidden behind his clinical, almost theatrical manner.
Despite the resources of a powerful patron and a state-of-the-art laboratory, Laurience remains the quintessential absent-minded professor—smoke curling from his fingers, eraser dust on the back of his wrinkled jacket, and a constant, almost comic need for a strong hairbrush. Yet the lure of Clare’s charm pulls him closer, and he Dares to reach beyond the bounds of his experiment by attempting to possess the body of Lord Haslewood's handsome son, Dick John Loder. In this new frame, he hopes to woo Clare in person, but the disguise cannot hide his strange, unmistakable physicality, nor can it mask the habit he keeps even in the presence of the young Haslewood—cigarette in hand, a habit Dick Haslewood never practiced.
A grim misstep soon follows: before transferring his mind into Dick, Laurience strangled Clayton, who was inhabiting Haslewood’s body, ensuring that Dick—now a prisoner inside Laurience’s own mind—would stand accused of the murder of the man believed to be his father. The web of deception tightens to a fatal knot, and the truth threatens to crush everyone caught in its folds.
Clare, aided by her ally Dr. Gratton Cecil Parker, senses the ruse and fights to restore order. They manage to return Laurience’s mind to its proper body, but the physical shell is not intact: the body has been badly broken in a panicked fall from a high window, a consequence of the chaos introduced by having Dick Haslewood inhabiting Laurience’s frame. Faced with the wreckage of his extraordinary invention and the moral weight of his selfish gambit, Laurience concedes that he should never have meddled with the human soul. The once-celebrated scientist chooses to abandon the dangerous knowledge he helped create, accepting the consequences of his actions and, with a final, sober realization, acknowledges that he has changed his mind for the last time.
