A small-town church organist named Scot Webster (Philip Terry) attempts to save his sister, Susan (Ellen Drew) from the clutches of big city gangster W. S. Bruhl (Paul Lukas). When one of Bruhl's gang members is double-crossed in Bruhl's rented room, and killed by a gunman, the man tosses him the gun and disappears. Scot is tried and executed. A scientist (George Zucco) salvages his brain and transplants it into a gorilla. Using the strength of his new, bestial body, Webster begins stalking the gangsters to exact his revenge.