[4] Inspired by the 1886 novella Strange Case of Dr. Jekyll and Mr. Hyde by Robert Louis Stevenson, the film follows the story of two American police officers visiting Victorian London who become involved in the hunt for a monster responsible for a series of murders.

While returning home from a pub, newspaper reporter Bruce Adams finds one murder victim, a prominent doctor.

The next day, two American policemen, Slim and Tubby, who are studying London police methods, respond to brawl at a women's suffrage rally in Hyde Park.

Reporter Adams, young suffragette Vicky Edwards, Slim, and Tubby are all caught up in the fray and wind up in jail.

Walking down a street at night, Tubby spots Hyde and the boys trail him into the music hall where Vicky is performing and Adams is visiting.

But before Tubby can bring the police inspector, Adams and Slim to the scene, the monster has reverted to the respected Dr. Jekyll.

Film Daily reported, "If the audience reaction at a sneak preview can be taken as a criterion, then Universal-International has another big treat for the Abbott and Costello fans.

"[4] The Los Angeles Times referred tongue-in-cheek to the liberties the script took with the original story: "If Robert Louis Stevenson is turning over in his grave, it's probably only so he can get in a more comfortable position for a belly laugh.