The Clue of the Rusty Key — Carolyn Keene

The Danas are thrust into the middle of a dispute between Oliver Pritz Gormly and Jasper Conway. The girls rescue Conway and his important papers when Gormly sets fire to Conway's store. Later, when Pritz confronts the girls and orders them to turn over the papers, the girls refuse, earning themselves a new enemy.

Jean and Louise learn that Gormly is a swindler, cheating many people, including their classmate Lettie Briggs, who refuses to believe that Gormly is dishonest. Jean and Louise can do little to help Lettie since she refuses to talk to the Danas, but the girls befriend several other victims as they search for clues.

The girls face many difficulties before they finally bring Dr. Gormly to justice.