In the Prologue, pirates hide a treasure on an island off the Mexican coast. And the pirates, the map and the treasure disappear in a volcanic eruption. The legend of the island draws treasure seekers for a few centuries, but the treasure lies undetected. Cut to 1938 and Carter Collins)---The Shark---takes possession of the island and turns it into a fortress, while he systematically seeks the gold. He takes over an ancient mansion, installs a physician, Doctor "X", his nurse Zanya, his butler, Hawkins, and Grindley, who has plans of his own, since he already has a portion of the map. The possessor of the other portion soon shows up. Meanwhile, Collins has his slave diggers---The Mole Men---digging up the island looking for the treasure. Back in the states reporter Larry Kent is assigned to go to the island to investigate the disappearance of another reporter, Paul Thorndyke. A sea captain who has half of the map is killed before he can tell the daughter, Toni Morrell, of an old shipmate about the other half but is around long enough to tell her that her father, that she doesn't know, is on the island but expires before he can tell her his name. Toni and Larry decide to join forces and solve the mysteries if the island.