The owner of a gold pharaoh's head clains it was aboard a frieghter Hardank but suspicions arise that he is trying to defraud theshipping l ine's insurance company. Frank and Joe investigate the legend of Whalebone Island when they receive a warning concerning a sunken freighter.
Frank and Joe solve the mystery of the secret locked room in the spooky Dalrymple Mansion. A banker named Ray Dalrymple of Lakeside gets death threats and shows up at the Hardy's house seeking Fenton Hardy's assistance. Frank & Joe offer to help, but at first he's reluctant. At last, he agrees. Soon after that, during a hike with their friends, the Hardy boys are nearly run over by a car, then investigate the Purdy place. After that, they rest and talk over their recent mis-adventures.
A plot to steal a top-secret instrument vital to the United States space program poses a challenging case to Frank and Joe Hardy. The whereabouts of their famous detective father is a mystery, so Frank and Joe are on their own to foil the plotters.
It all starts when Fenton Hardy, is engaged by an experimental race car and motor designer to investigate a series of mysterious accidents. Three of his drivers have crashed because the windshields of their cars were suddenly crazed, thus cutting off forward vision. Frank and Joe uncover one slim clue. Each of the drivers had seen a signpost marked 'Danger' shortly before the accident.
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 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, chief pilot for Stanwide, was killed? Frank and Joe launch an aerial search for clues to the platinum thieves' hideout, believing that they will also uncover the mystery behind Hill's accident.
