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Frank and Joe Hardy are determined to bring to justice the vicious criminals who kidnapped their detective father while he was investigation sabotage of a road-building project in the Kentucky wilderness. After Mr. Hardy's escape, the two young sleuths are spurred into action when he mumbles the word "Felix” and they discover a dossier of a notorious ex-convict was stolen from his files. The trail leads them to New York City but ends abruptly in a cemetery! With the new clue of a spiral symbol the boys head to Kentucky as part of a highway construction crew to track down the kidnappers and saboteurs.

When dogs and men suddenly disappear, and strange screams fill the night, fantastic stories of vengeful ghosts are almost believable. It is these strange happenings which bring Frank and Joe Hardy to the Pocono Mountains to help their father's friend solve the mystery of Black Hollow. But when the Hardy boys and Chet Morton arrive at Captain Thomas Maguire's cabin on the edge of the hollow, he has disappeared. Frank and Joe are determined to find the captain, despite Chet's misgivings after a night of weird and terrifying screams. Strangely, it is a small puppy that discloses a most unusual and surprising set of circumstances, involving a mute boy, an elusive hermit, and a fearless puma trainer.

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.

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.

There is a crisis in Bayport! Soon the city will be without water! Every night, water strangely disappears from the new reservoir near Skull Mountain. Frank and Joe Hardy join forces with a team of skilled engineers to solve the baffling mystery.

Frank and Joe race against time to retrieve a scientist's top-secret invention before thieves get their hands on it. But the young detectives soon discover that they are involved in a mystery far greater in scope than just safeguarding the invention. When Joe is kidnapped, this incident starts Frank of on a chase that could end disastrously for him and his pals.

Triggered by the disappearance of a brilliant young professor, this mystery takes the Hardy boys first to Kenworthy College, where they find a puzzling message on an examination paper. But then, a new clue is unearthed that sends the young detectives to the Honeycomb Caves. After many dangerous encounters, Frank and Joe discover a powerful searchlight in one of the craggy caverns along the seacoast. In the brilliant white glare of the searchlight, the startling secret of the caves is dramatically revealed.

When the Hardy Boys eagerly agree to assist their friend Jack Dodd and his father in locating a family treasure, the two young detectives are unaware of the baffling complications that will arise. Deadly road traps, aerial ambush, a spider man and a suspect who carries a menacing cane harass the young detectives as they seek the triple solution to this perplexing and exciting mystery.