A long-distance telephone call from Chet Morton's uncle summons Frank and Joe Hardy and their staunch pal Chet to a summer art school, located near old Fort Senandaga which is reputed to be inhabited by a ghost. The young detectives' assignment: recover two famous oil paintings stolen from the valuable Prisoner-Painter collection owned by Jefferson Davenport. Mr. Davenport reveals that one of the famous pictures contains a clue to the hiding place of a priceless gold chain. Vicious threats and deadly traps beset Frank, Joe, and Chet as they search for clues to the stolen paintings and the gold treasure - a search that is complicated by the stormy feud between an Englishman and a Frenchman over the military history of the ancient fort.