Two young relatives join a retired yachtsman on a southern cruise and find an old chest whose contents point to hidden treasure near a Caribbean inlet. The voyage shifts into a hazardous treasure hunt marked by storms, shipboard treachery, and a mutinous faction. Stranded parties, wreck investigations, and explorations of caves with secret passages, traps, and an underground treasure chamber propel the action. Ingenious schemes, armed confrontations, and a timely rescue determine the fate of the hoard and bring the expedition to its conclusion and departure.