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 detective's astonishment it proves to be a wallet containing two thousand dollars. The next day their close pal Chet Morton rushes to the Hardys' home to report that the Morton farm truck carrying a shipment of high-powered rifles belonging to his uncle, a big-game hunter, has been stolen. These two apparently unconnected events are clues in a tangle of mystery which turns out to be one of the detective brothers' most exciting adventures.