In the gay '90s, cardsharps take over a Mississippi riverboat from a kindly captain. Their first act is to change the showboat into a floating gambling house. A ham actor and his bumbling sidekick try to devise a way to help the captain regain ownership of the vessel.

Dr. Paul Carruthers is frustrated because he thinks his employers, Mary Heath and Henry Morton, have cheated him out of the company's profits. He decides to get revenge by altering bats to grow twice their normal size and training them to attack when they smell a perfume of his own making. He mixes the perfume into a lotion, which he offers as a gift to Mary and Henry. When they turn up dead, a newspaper reporter decides to investigate.