When Mr. Hardy disappears while investigating a mystery surrounding a vacant house rumored to be either haunted or an abode for criminals, Joe and Frank search for the truth.
Joe and Frank suspect there is a link between the counterfeiting case they are investigating, and the case their father can't discuss. Determined to learn the secret, the Hardy Boys get themselves into a scrape.
During their search for members of a notorious gang responsible for a payroll robbery, Joe and Frank Hardy encounter timber wolves, a Rocky Mountain blizzard and a mine cave-in.
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 a stormy feud between a proud Englishman and an equally proud Franchman over the military history of the ancient fort.
Helping in a case their father is working on, the Hardy brothers try to find an informer who has telephoned from a booth in their home town, and are hired by a carnival owner to spot pickpockets.
While attending an amusement park under the new ownership of their friend Daisy, Joe and Frank investigate the disappearance of a patron who was last seen riding a hit attraction, an event that has threatened Daisy's family's livelihood.
While searching in Iceland for a missing sailor sought by an insurance company, the teen-age detective brothers uncover a plot threatening the life of a United States astronaut and the secrets of NASA's moon project.