Gideon Bell, owner of the Favre Emeralds, receives a letter stating that the emeralds will be stolen at midnight by "the Bat", and that police will not be able to stop the robbery. The Bat, a figure dressed as a bat, murders Gideon and steals the emeralds. The Bat leaves a Calling card (crime) for the chief of police to inform him that he will be traveling to the country. The Bat travels by car to a mansion built by Courtleigh Fleming, the president of the Oakdale Bank, who has recently been found dead in Colorado. The mansion is being rented for the summer by writer Cornelia Van Gorder, whose maid, Lizzie Allen, sets up a Trapping to catch the Bat. Richard Fleming, Courtleigh's spendthrift nephew, wishes to lease the mansion, and plans with Dr. H. E. Wells to frighten Van Gorder away.