Lieutenant Allerton of the U.S. Navy obtained a 3-month leave and he, along with a Mayan named Chaka, ask Captain Steele for passage on the Seagull to Yucatán. Allerton's mother and sisters are about to lose their mortgaged home, and Chaka knows of a hidden city where people from the lost continent of Atlantis live. They have gold and rubies that mean nothing to them, and Allerton wants to trade modern items to them for the precious items that can help his family keep their home. Sam and his friends Joe and Archie volunteer to go with him. They are joined by Ned Britton, Nux and Bryonia. Native tribes, including Chaka's people, are a danger, for they hate white people, but Allerton's uncle is an inventor who gave him wonderful items for protection and defense, such as an electrical device that knocks enemies unconscious. It is a perilous adventure, the boy fortune hunters and their companions are nearly sacrificed to the god of the sun, but they escape with treasure, and Lieutenant Alberton saves the family home.
