Two bumbling American cops in turn of the century London hunt for the monster"Mr.Hyde"..unaware that he is really "Dr.Henry Jekyll"(Boris Karloff)who has turned himself into the murderous creature with a serum...One of the cops"Tubby"(Costello)inadvertedly gets an injection of the drug and turns into a monster..which sends his pal"Slim"(Abbott)and all of Scotland Yard's officers on a zany chase after the real demon..Karloff and Costello don't appear in the final chase scenes..stunt men:Eddie Parker and Vic Parks perform those stunts.