A group of scientists arrive on a remote Pacific island to investigate what became of the previous team, which was sent out there as observers of American hydrogen bomb tests and disappeared without a trace. They island is uninhabited and devoid even of most animal life, except for a few land crabs. The group's arrival is marred by the death of one of the navy crewmen accompanying them, who falls into the ocean and comes up with his head taken off. Then the navy plane that carried them there is blown out of the sky before it can get airborne, leaving them stranded and unaccounted for. Led by Dale Drewer (Richard Garland) and Dr. Karl Weigand (Leslie E. Bradley), they find the journal of the previous team, but no explanation of what happened to them, only that they'd noticed evidence of strange creatures and inexplicable physical phenomena on the island. Soon the scientists are hearing the voices of members of the previous scientific party, calling to them in the night. Their own radio is sabotaged and something has been probing the area where they're living; finally, the group is lured into the caverns where the real menace is hiding -- gigantic, bloodthirsty mutated land crabs that communicate telepathically and seem to have all of the knowledge of the previous team's members. One by one, all but three of the members -- Brewer, electrical engineer Hank Chapman (Russell Johnson), and scientist Martha Hunter (Pamela Duncan) -- are killed off and their minds and memories absorbed by the mutant crabs, who also have the power to focus infrared radiation into deadly, destructive beams that they use to gradually pulverize large sections of the island. By the end, a single giant crab has run the trio to ground on a remaining parcel of land just a few dozen yards across.