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.

Frank and Joe set out on a search for the men who kidnapped their father and for the dossier of an ex-convict that was stolen from him.

When the Hardy Boys take a winter vacation in Jamaica, Joe finds an ancient bronze death mask washed up near their beach house during a violent storm. Helping the Hardys and their friends in this bizarre mystery is William, a Jamaican boy, who flies to New York with startling news, only to be intercepted and held for ransom—the death mask! Frank and Joe must rescue William, plunge into their father’s airline-ticket theft case, and fly into a maze of danger in Africa.

Another exciting mystery begins for Frank and Joe when they help a stranger who has had an accident with his car. The man introduces himself as John Mead, owner of a nearby estate. But when the Hardy boys find an odd-looking house key and try to return it, they realize that John Mead died five years ago! They are even more amazed when they find that the intricately carved doors in the deserted mansion have no visible knobs or keylocks. What happens when Frank and Joe discover that there is a link between their father’s case of thieves and the mysterious Mead mansion will keep the reader on edge with thrills and suspense.

One exciting event follows another when Frank and Joe Hardy are hired to apprehend the pickpockets who have been plaguing Solo’s Super Carnival. When their friends Tony Prito and Biff Hooper exhibit a stuffed whale dug up at a construction project, they all but put the carnival out of business. In this thrilling mystery the young detectives pit their wits against a gang of thieves whose bizarre identification, a three-part whale tattoo, proves to be a nearly insolvable riddle.

During Mr. Hardy’s investigation of a ruthless gang of terrorists, Frank and Joe witness an explosion in the sky near an airborne dirigible owned by Quinn Air Fleet. The young detectives look into a clue that takes them into a new animal park outside Bayport, where they are lured into a trap by an unknown enemy. Problems arise for the park owner as he receives pressure from a competitor and a real-estate firm to sell out. Strange occurrences at the park also frighten the visitors and animals. Frank and Joe take up the case despite fore warnings.