As two American cops sent to London to study crowd control, Abbott and Costello inadvertently become involved with mad scientist Dr. Henry Jekyll (Boris Karloff), who has been busy transforming animals into weird hybrids (a cat that moos like a cow, a rabbit that barks like a dog) and
murdering colleagues who have mocked his theories. Jekyll does this by injecting a special serum that turns him into the evil Mr. Hyde. (Stuntman Eddie Parker played the monster for the 66-year-old Karloff in the more athletic scenes.) Of course, comedic chaos ensues as Jekyll/Hyde becomes more
crazed and decides to kill the Americans. In the climactic chase, Costello is jabbed with a...