While experimenting on his own body with spinal fluid from a gorilla, Dr. James Brewster (Bela Lugosi) accidentally turns himself into a hairy ape-man. The only way for Brewster to return to normal is to capture a human and extract spinal fluid from its body. With the help of his gorilla sidekick (Emil Van Horn), the deformed Brewster begins hunting for donors, which quickly creates panic on the streets.