Rarely do magicians reveal their professional secrets. Consequently Frank and Joe Hardy are amazed when a well-known magician, the Incredible Hexton, offers to reveal the secret of his “Vanishing Man Act” and invited Mr. Hardy to be the subject. When their detective father fails to reappear, his sons are convinced that something sinister is afoot, despite Hexton’s insistence that Mr. Hardy is playing a joke on them. While desperately searching for their father, Frank and Joe find themselves working with SKOOL, a U.S. organization of secret agents pitted against UGLI, an international ring of spies stealing government secrets domestically and internationally. The young detective’s gripping adventures culminate in a dramatic climax when they unmask the secret agent on Flight 101.

Tony Prito enlists the help of his detective friends Frank and Joe Hardy when a sinister stranger demands that Tony sell him the bizarre curio collection he has just inherited. While the boys are discussing this suspicious incident, the stranger, Valez, telephones and threatens Tony. That same afternoon the three boys collect the cases of curios at a freight station only for Joe’s life to be at risk on their way back to Tony’s house. The next day a seaman claims two medallions in the collection that coincidently go missing. Did Valez steal them? And what was their significance. Unraveling the clues in this exciting mystery takes the Hardys and their friends to a desolate region in Guatemala and straight into the hands of a gang of dangerous thugs.

A ferocious, wolf-like creature appears during the night of the full moon in a small Adirondack town, attacking livestock and scaring many of the residents. A young man with a history of werewolves in his family line is suspect. Joe barely escapes a horrible death as the young detectives solve this exciting and hair-raising mystery.

A cryptic message from their famous detective father and a note concealed in a ventriloquist’s dummy lead Frank and Joe Hardy on a dangerous search to the tropical islands in the Caribbean. There the teenage detectives are constantly beset by vicious henchmen of a criminal mastermind. Danger stalks the boys’ every move, once in an isolated sugar mill, another time in a shark-infested sea. But when Frank and Joe come face to face with the ghost at Skeleton Rock, it will be as much of a surprise to the reader as it was to the young detectives themselves.

With only the slender clue of an arrow-shaped tie clasp, Frank and Joe Hardy pick up the trail of a cunning gang of thieves responsible for a wave of jewelry-store holdups. But their investigations are interrupted when a desperate plea for help comes from their widowed cousin who lives on a cattle ranch in New Mexico. Frank, Joe, and their pal Chet fly there immediately, and manage to put an end to the trouble at the ranch and solve the jewelry-store robberies.

An assignment form their famous detective father to track down a ring of credit-card counterfeiters takes Frank and Joe Hardy on an exciting camping trip to the Rocky Mountains. But the cross-country trek with their pals Chet Morton and Biff Hooper is jinxed from the very first day. Strange happenings on a nearly impassable mountain lure Frank, Joe, Chet, and Biff to almost certain death before they discover the sinister reason for the danger on Vampire Trail.

A plot to steal a top-secret instrument vital to the United States space program poses a challenging case for Frank and Joe Hardy. After rescuing a South American stowaway who mutters a warning about “Footprints “ and then vanishes, Frank and Joe discover that documents belonging to their famous detective father are missing and that footprints linger under a window. The young detective’s 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.