A plot to steal a top-secret instrument vital to the United States space program poses a challenging case for Frank and Joe Hardy. The whereabouts of their famous detective father is a mystery, so Frank and Joe are on their own to foil the plotters. After rescuing a South American stowaway who mutters a cryptic warning about "Footprints," then flees, the boys learn they are up against a ruthless espionage ring. The next day Frank and Joe discover that documents belonging to Mr. Hardy are missing, and, noting strange footprints under a window, they suspect the stowaway. The young detectives' search for the diabolical mastermind of the "Footprints" spy ring takes them on a flight to a group of islands off the coast of South America. Here they risk the wrath of a cruel dictator, and also make a grisly discovery deep in the jungle. During a spine-chilling vigil in a cemetery, followed by entrapment in the flooded compartment of a wrecked ship, the courage and resourcefulness of the Hardys and their pal, Chet Morton, are tested to the utmost. Hardy boys fans will find this exciting tale crammed with suspense, intrigue, and action. - Flyleaf.