The Hardys purchase a Chinese junk to use as a ferry service, in order to make some money over the summer. It soon becomes apparent that others are very interested in the ship.

A sleek new hydrofoil is scheduled to start ferrying passengers between Bayport and Cape Cutlass. But business enemies of the hydrofoil owner have stirred up a hornets’ nest of violent opposition among small boat owners. Fearing sabotage, he begs Frank and Joe Hardy to guard the Flying Express on her maiden trip. Startling developments plunge the teenage detectives into a dangerous chase by sea, air, and land in pursuit of a gang of hardened criminals who operate by the signs of the Zodiac. Tension mounts when the Flying Express vanishes – and so does Sam Radley, Mr. Hardy’s skilled operative. Peril stalks Frank and Joe’s every moves as they hunt down the terrifying gangleader Zodiac Zig and his vicious henchmen.

With two mysteries to solve, Frank and Joe must climb aboard their iceboat the Sea Gull to reach Cabin Island, where a belligerent stranger orders them off. Sabotage to the boat, danger to themselves, and a ghostly prowler do not daunt the Hardy boys in their search for Johnny Jefferson and for clues to stolen antique medals. How the teenage investigators outwit a ruthless foe and succeed in solving both mysteries makes for mounting suspense in this brisk-paced adventure.

The handwritten will of a deceased world-traveler is strange and mysterious. Its cryptic instructions are to deliver “the valuable Aztec warrior to the rightful owner, a descendant of an Aztec warrior.” Frank and Joe Hardy have only one slim clue to work with: the name of a complete stranger who can help. Despite the harassments, the threats, and the attacks made upon them, Frank and Joe unravel clue after clue in their adventure-packed search for the living descendant of the mighty Aztec nation which once ruled in Mexico. It takes as much high courage as clever deduction for the young detectives to defeat their ruthless foes and to decipher the fascinating secrets of the strange and mysterious will.

Strange events involve Frank and Joe Hardy in a mystery which shrouds an ancient treasure—the golden head of the Pharaoh Rhamaton IV. The owner of the million-dollar golden Pharaoh’s head claims it was aboard the freighter Katawa, which sank not far from Whalebone Island. But suspicious developments indicate that Mehmet Zufar may be trying to defraud the shipping line’s insurance company. Frank and Joe enthusiastically accept the challenge of their famous detective father to assist him in investigating the complex case for Transmarine Underwriters.

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.

The famous young detectives Frank and Joe Hardy are caught up in a dangerous web of intrigue when they agree to help Greg Woodson search for his grandfather’s missing will. When Henry Kurt, the temporary headmaster of Woodson Academy, insists that he is to inherit the property from Greg’s grandfather, the case becomes even trickier. Frank and Joe must risk their lives several times before they solve the mystery of Yellow Feather and trap a sinister criminal who will stop at nothing--even murder--to satisfy his greed for money.