The Monster and the Girl (1941)

Scot Webster tries to save his sister Susan from the clutches of gangster W.S. Bruhl. When Scot comes to Bruhl's rented room, one of the gangster's henchmen collapses into his hands, killed by a gunman. The murderer tosses his gun to Scot and disappears. Since all the evidence points at him, Scot is arrested, tried and sentenced to death. A mad scientist uses his brain to transplant it into a gorilla. After the operation Scot wakes up in the body of a gorilla, eager to get his revenge...