Jack Castle was both a drunk and assistant to a mad scientist. He is knocked unconscious by the mad doctor, so Castle would not reveal the nature of the doctor's experiments to Elsa, the doctor's niece. Castle's brain is removed and then placed into the body of a gorilla. When he awakens from his operation, he seeks revenge. First, he kills the mad doctor, and then goes after his assistants hoping to be cured of his affliction.