Stanley Ridges, playing "Prof. Kingsley," suffers from a grave illness, which leads Boris Karloff ("Dr. Sovac") to carry out an improper procedure involving brain transplantation—specifically, replacing his friend’s brain with that of the gangster “Red Cannon.” The operation succeeds in keeping Ridges alive, but the Professor starts exhibiting several undesirable qualities associated with "Cannon." "Sovac" discovers from one incident that a sum of $500,000 is concealed somewhere and subsequently tries to influence his companion into disclosing its whereabouts. Advertised as a film featuring both Karloff and Lugosi, changes in casting resulted in them not sharing any scenes together; however, this isn't particularly significant because the movie functions more effectively as a science fiction thriller than a horror story, and neither actor delivers a truly memorable performance. Ridges is the focus of attention, managing to portray his dual identities with skill, while Arthur Lubin maintains an adequate pace for the narrative. Ultimately, though, it feels rather ordinary, adhering to a conventional plot and resolution.

Source on TMDB