Two American police officers, Slim and Tubby, go to London to study police methods. They meet a musical performer, Vicky, and her guardian, Dr. Jekyll, who is secretly experimenting with transplanting personalities. Jekyll turns himself into a monstrous creature, Mr. Hyde, who terrorizes London and tries to murder Vicky's fiancé. Tubby captures Hyde, but he turns back into Jekyll and is released. Slim and Tubby hunt for the monster in Jekyll's home, where Tubby accidentally turns into a monster himself. Hyde kidnaps Vicky and is chased back to Jekyll's home, where he falls to his death. Tubby eventually turns back to normal after biting several policemen who also become monsters.