A group of scientists and their support crew of five sailors land on a remote island in the Pacific, driven by two goals: to locate a missing previous expedition and to continue studying how Bikini Atoll radiation affects local plant and sea life. The team is led by Dr. Karl Weigand, Leslie Bradley, and includes geologist James Carson Richard H. Cutting along with biologists Jules Deveroux Mel Welles, Martha Hunter Pamela Duncan, and Dale Drewer Richard Garland. Their party also features technician and handyman Hank Chapman Russell Johnson. As soon as they land, danger rises quickly: a sailor named Tate is pulled into the sea and decapitated, his body left floating—an ominous first sign that something aboard the island is not as it seems. Two sailors remain behind to guard the researchers, while the others, with Ensign Quinlan Ed Nelson at the helm, attempt to return to the mainland, only for their seaplane to inexplicably explode. A storm prevents the team from reporting back, so they stay and press on with their work, determined to understand what happened.
Inside the makeshift camp, the scientists study journals from the prior expedition, which speak of killer worm-like creatures. Martha and Dale later go underwater for scuba diving to collect specimens, and that night Martha is startled by a voice—McLane, the former expedition’s leader—calling to her from the dark. A sudden, unexplained earthquake opens a pit, and Carson slips into it, fighting to regain his footing. As the team digs deeper, they uncover a chilling truth: the earlier group was killed and eaten by two mutated, intelligent giant crabs. These creatures have absorbed the minds of their victims and can communicate telepathically in those voices, lending them an unsettling sense of consciousness and intent. The current expedition soon realizes that the crabs have become virtually invulnerable to ordinary weapons because of cellular mutations, and the pair begin to systematically pick off the scientists one by one.
From this point, the team pieces together the crabs’ grand plan: the tremors and landslides are not random but caused by the same creatures, who are undermining the island with tunnels and slowly eroding the land itself. The scientists pivot to stopping the mating pair and halting reproduction, hoping to save what remains of the island and possibly avert a larger catastrophe. In a tense rescue, they manage to kill one crab in a cave when an explosive causes an overhead rock to collapse, crushing the monster’s head. With the island continuing to crumble into the Pacific, the researchers barely escape their collapsing laboratory building, and three survivors—Dale, Martha, and Hank—come face-to-face with the remaining intelligent giant crab, Hoolar, who communicates via telepathy. Hoolar reveals a chilling desire to travel to the mainland with fertilized eggs to feed on more minds, even after the island vanishes and the humans are gone. In a final act of sacrifice, Hank triggers an electrically-charged broadcast tower to topple onto the giant crab, electrocuting both the monster and its unfertilized brood. As the island’s last remains slip beneath the waves, Dale and Martha cling to each other, watching the land disappear while their world—once a lab, then a battlefield, now a fragment of rock and ocean—drifts toward an uncertain horizon.
