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.

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.

Not only is the local bank robbed, but two of the Hardy boys' pals mysteriously disappear after a masquerade party. Are the events related? How the Hardy boys use all their courage and skill to outwit the criminals provides an exciting climax to one of the most baffling mysteries the young detectives have ever encountered.

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.

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.

First valuable antique weather vanes are stolen in Pennsylvania Dutch Country and Frank and Joe are hired to catch the thieves. Meanwhile, their father is asked to find a missing Navy employee and a top-secret government document. What is the connection between the missing weather vanes and the Pentagon spy? The final chapters of this thrilling mystery uncover a stunning revelation!

Frank and Joe Hardy become involved in a case affecting national security when Joe dials a wrong telephone number and gets the Pentagon. Two words— “Bombay Boomerang”— that the boys hear before the line goes dead plunge them into a whirlpool of danger and intrigue. At the same time, Frank and Joe must save their father from a murderous gang stealing mercury shipments in the Baltimore harbor. With clues linking the mercury thefts to the top secret Super S missile mysteriously stolen from a government arsenal. In a race against time, the three Hardys foil a diabolical scheme to create widespread havoc in the United States.

A series of museum thefts launch the Hardy Boys on this baffling mystery. Rare collections of ancient pirate treasure are being stolen, so Frank and Joe are asked to stake out the Black Parrot, a suspicious freighter docked in Bayport Harbor. This whirlwind chase after a self-styled pirate king in the Caribbean stronghold of the Empire of the Twisted Claw makes a gripping tale of suspense and high adventure.

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.