When his best friend, an absent minded professor (Stanley Ridges), is hit by a car driven by a gangster, a scientist (Boris Karloff) does a brain transplant. In order to save his friend's life, he gives him the gangster's brain. But this causes a Jekyll and Hyde personality as the mild mannered professor and the murderous thug fight for dominance of the body. While entertaining, this is a poorly constructed movie. Whether scientist or gangster, most of the characters behave stupidly. For example, when gangster Bela Lugosi (a minor role, but a nice change of pace role for him) holding a gun orders an unarmed Ridges to give him a case containing money, he says "Okay, come and get it". Instead of telling him to put down the case and move away ... Lugosi walks over to take the case! You can guess what happens. Also, inexplicably Karloff changes from a benevolent doctor to money hungry and manipulative crook without much explanation. Though Karloff and Lugosi are top billed, the movie belongs to Stanley Ridges. Without much make-up at all, he manages to shift between the professor and the thug smoothly and so believably that you'd almost never guess it was the same actor! Directed by Arthur Lubin (BUCK PRIVATES). With Anne Nagel, Anne Gwynne, James Craig, Paul Fix and Virginia Brissac.