A radio newscast announcing a reward to anyone who can locate a valuable silver dolphin involves Louise and Jean Dana, teen-age sister sleuths, in a baffling search. Pretty Judy Platt is desperately trying to find the silver dolphin mentioned in her dead brother's will. But no one knows what kind of dolphin to look for -- a live mammal or a silver object. The only clue is Oliver Platt's signature, encircled by four stars to represent the constellation Delphinus -- the dolphin. The Danas' sleuthing is hindered by a hostile fortuneteller and a hoarse-voiced man wanted by the police.

It soon becomes apparent to the young detectives that Maria Castone and "Throaty" Sensky are engaged in a sinister conspiracy to get the reward, and will stop at nothing to ensure the success of their criminal scheme. Louise and Jean's quest for the silver dolphin leads them and Judy Platt to Miami, Florida, and then to a deserted Caribbean island called Job's Coffin.