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