The robbery of the treasure in the tower proves to be an exciting challenge to Frank and Joe. Frank and Joe attempt to foil a ruthless espionage ring in their effort to sabotage the United States space program.

The handwritten will of the 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." What is the valuable object and where is it? What is the name of the owner and where is he? Frank and Joe Hardy have only one slim clue to work with: the name of a complete stranger who can help find the answers. Despite the harassments, the threats,...

Valuable electronic parts containing platinum are being stolen from shipments made by Stanwide Mining Equipment Company's cargo planes, and Frank and Joe Hardy are called upon to assist their world-renowned detective father to solve the baffling case. At Stanwide the boys pose as employees, and become suspicious of their boss's hostility toward them. Is he involved in the racket? And what is the truth behind the plane crash at sea in which Clint Hill,...

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 thteam of skilled engineers to solve the baffling mystery. When the boys arrive at the foot of the mountain, they are met by a human skull rolling down the slope -- a chilling warning from a weird hermit. Violence, too, stalks the mountain, where a gang controlled...

Frank and Joe, the Hardy boys, solve a mystery involving a magician, American secret agents, and international spy ring, and a Scottish castle.

Caught in their motorboat the Sleuth by a sudden storm at sea, Frank and Joe Hardy are helpless when the engine conks out. Drifting farther from shore amid the tumultuous waves, the boys are relieved to see the lights of a yacht. But their SOS is ignored. The shrill sound of a siren fills the night air. Moments later, a helicopter drops an object alongside the Sleuth. To the young detectives' astonishment it proves to be a wallet containing two thousand...

A chance meeting with a stranger on the road provides the Hardy brothers with a clue to a gang of local burglars.

Suspecting that their friend has been swindled, the Hardy brothers investigate and find themselves on the trail of a much larger criminal operation.

When two suspicious plane accidents occur near an eastern airport, the two Hardy brothers investigate the case and find themselves in greater danger than they anticipated.