A rash of murders (by an unknown "monster") is plaguing London, and police are baffled. A newspaper reporter, Bruce Adams (Craig Stevens (actor)), finds one of the murder victims while coming home from a bar at night and calls the police. The next day, two American policemen, Slim (Bud Abbott) and Tubby (Lou Costello), who are working for the London Police Force, respond to a mob fight at a Women's Suffrage Rally in Hyde Park. Reporter Adams, young suffragette Vicky Edwards (Helen Westcott), Slim, and Tubby, all get caught up in the fray and wind up in jail. Later, Vicky's guardian, Dr. Henry Jekyll (Boris Karloff), bails Vicky and Adams out. Tubby and Slim are thereafter kicked off the police force. Unknown to anyone, however, Dr. Jekyll has developed an injectable serum which transforms him into Mr. Hyde (the "monster" responsible for the recent murders). When Jekyll notices Vicky's and Bruce's mutual attraction, he has more thoughts of murder, injects himself, and transforms once again into Hyde (with the intent of murdering Adams).