The provided text chronicles the sixth adventure of The Three Investigators—Jupiter, Pete, and Bob—who are sent to Skeleton Island under the guise of acting in a film, but secretly tasked with investigating sabotage affecting the movie crew. The boys' arrival is fraught with trouble when a man named Sam maroons them on the smaller island, The Hand, a mistake rectified by the timely rescue of a local boy, Chris Markos, who is widely and unfairly suspected of being a thief. While searching for clues, the investigators stumble upon an underwater cave containing pirate gold, a discovery that forces the thieves, who were using the phantom ghost story and sabotage to keep people away from their hidden bank robbery loot, to make a move. The narrative concludes with Jupiter deducing that the island’s "secret" is the money stolen ten years prior by the Ballinger brothers, with Chris helping to capture the criminals, ultimately clearing his name.