Dr Claire Wyatt's colleagues are disappointed that she will be leaving them to work with the brain specialist, Dr Lorenz. They, and her fiancé, Dick Haselwood, try to dissuade her, teasing her that Lorenz is mad. Nonetheless, she goes against their wishes, and sets off to leave the next morning.
She meets Lorenz and his wheelchair-bound assistant, Clayton, at their laboratory. The next day, Dick turns up and tries to get Claire to leave again. She manages to reassure him that she is all right, but as he departs he hands her a copy of a newspaper, in which he has written an article on the 'secret activities' taking place where she is now working.
Lorenz is incensed by the story, and is spurred on to show Claire the fruits of his research. Two monkeys are brought into the room, one docile and the other violently aggressive. Lorenz attaches them to his apparatus and swaps their minds. In his excitement, Lorenz suggests the possibility of performing the experiment on humans, but Claire is against it.
Meanwhile, Lord Haselwood (Dick's father, and owner of the newspaper in which Dick's article was published) thinks that reports of Lorenz's experiments will sell papers and arranges a visit to meet him. Haselwood is the benefactor of the Haselwood Institute, a scientific research foundation, and he offers Lorenz funding and equipment in return for exclusive rights to Lorenz's finished work.
Haselwood pressurises Lorenz into presenting his findings early, to a hall packed with the leading British scientists of the day. The audience members mock his theories, and Lorenz makes a fool of himself, proclaiming the validity of his research as they file out of the building.
Haselwood blames Lorenz for the whole debacle, and withdraws his funding. This pushes Lorenz's unstable mind over the edge, and he tricks Haselwood into sitting on the thought transference chair. He then proceeds to swap Haselwood's mind with that of Clayton's, killing Haselwood in the process. Lorenz tells Clayton (who is now in Haselwood's body) that if he pretends to be Haselwood then he can continue the funding for Lorenz's experiments.
However, Clayton discovers that Haselwood's body has a serious heart condition, and in a panic he contacts Lorenz with the news. In the meantime, Clayton has dreamt up a solution to the problem. He has made out his will so that all of his money will pass on to Haselwood's son when he dies. All that remains is for Lorenz to transfer his mind into the body of Dick, and then they can carry on as normal.
Instead, Lorenz strangles Clayton/Haselwood, and resolves to swap his own mind with that of Dick, so that he can get the money and win Claire's affections. By leaving enough evidence, Lorenz will be tried for murder - but it will be Dick's brain in Lorenz's body.
Dick arrives at the institute, and Lorenz drugs him, conducts the experiment and exits the machine in Dick's body. He pours some chemicals under the door of the chamber where Dick/Lorenz is sat, and the vapours from them begin to suffocate him. Claire arrives and is pleased to see what she assumes is Dick, but she recognises Lorenz's physical traits and realises what has happened.
Dick/Lorenz breaks out of the machine and in an attempt to get some air, smashes through the window, falling to the ground outside. The police arrive to arrest Lorenz for murder, and Claire gets them to restrain Lorenz/Dick.
She runs out of the building and sees Dick/Lorenz lying on the floor. The doctor tells her that he has only a few minutes left to live, and she manages to persuade him to bring the body upstairs and perform the thought transference experiment again, with Claire taking the controls.
They swap minds successfully, and Lorenz, back in his real body, implores Claire to destroy the equipment, having finally decided that the mind is sacred. He dies, and Claire and Dick are reunited.
