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.

In solving the mystery of two medallions missing from an inherited curio collection, the Hardys wind up in a desolate area of Guatemala at the mercy of dangerous thugs.

Chet Morton's cousin, Vern, is on his way to California to find a rare and valuable coin mysteriously missing from his uncle's bank vault. When he stops in Bayport, his brand-new car is stolen. The Hardys take on a double mystery-and double danger as they head for the West Coast to investigate this sinister mystery.

A dying criminal confesses that his loot has been stored "in the tower." Both towers of the looted mansion are searched in vain. It remains for the Hardy boys to make an astonishing discovery that clears up the mystery and clears the name of a friend’s father.

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.

A long-distance telephone call from Chet Morton’s uncle summons Frank and Joe Hardy and their staunch pal Chet to a summer art school, located near old Fort Senandaga which is reputed to be inhabited by a ghost. The young detectives’ assignment: recover two famous oil paintings stolen from the valuable Prisoner-Painter collection owned by Jefferson Davenport. Mr. Davenport reveals that one of the famous pictures contains a clue to the hiding place of a priceless gold chain. 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 the stormy feud between an Englishman and a Frenchman over the military history of the ancient fort.

Danger is the name of the game when the Hardys agree to help their pen pal from Greece, Evan Pandropolos, search for a priceless, ancient Greek helmet. Years ago, Evan’s uncle had loaned it to Hollywood movie company for use in a silent motion picture, but the treasured helmet had been lost. Picking up foes along the way, the young detectives unearth clues that keep them constantly on the move—from their Greek college campus to California and finally to Greece.