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 towards 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. The puzzling trail of clues leads the young sleuths to an uninhabited Caribbean island, near the scene of the plane crash - then to a mountaintop in Montana and a danger-filled showdown with the band of thieves. But the final discovery - and the most startling and exciting revelation of all - is made in the boys' hometown of Bayport.