Dr. James Brewster and his colleague Dr. Randall are involved in a series of scientific experiments which have caused Brewster to transform into an ape-man. In an attempt to obtain a cure Brewster must inject himself with recently drawn human spinal fluid. Reporter Jeff Carter and photographer Billie Mason are on assignment (initially suggested by an odd man) investigating the recent disappearance of Dr. Brewster.
Before interviewing Brewster's sister Agatha, a "ghost-hunter", they hear strange sounds outside the house. After Dr. Randall's butler is murdered and the only clue is a fistful of ape-like hair, Carter deduces that the ghostly sounds they heard may well have been from an ape. Carter returns to investigate further. Dr. Randall informs Agatha that he will not help her brother again – and will go to the police if necessary. Needing more of the fluid as its effects are only temporary, Brewster and his ape go on a killing spree (the odd character appears yet again – saving one of the potential victims).
Brewster returns to Dr. Randall demanding he inject the fluid. When Randall breaks the precious vial on the doctor's floor, the enraged Brewster strangles him. Carter and Mason return to Brewster's home separately. While cautiously investigating, Billie knocks Jeff unconscious. Dr. Brewster then carries the photographer off to his basement lab – to again withdraw more spinal fluid. Carter regains consciousness and while he and the police attempt to break into the secret basement entrance, Brewster is attacked by the ape. The ape breaks Brewster's back, killing him.