The Secret of the Silver Dolphin

Teenage sister sleuths Louise and Jean Dana are drawn into a puzzling search for a valuable silver dolphin after a radio newscast announces a reward. They team up with Judy Platt, who desperately needs to find the dolphin mentioned in her late brother's will, but no one knows if it's a living creature or an object. Their quest takes them from Miami to a deserted Caribbean island, all while evading a hostile fortuneteller and a suspicious man determined to claim the reward for themselves.