A group of scientists travel to a remote island to study the effects of nuclear weapons tests, only to get stranded when their airplane explodes. The team soon discovers that the island has been taken over by crabs that have mutated into enormous, intelligent monsters. To add to their problems, the island is slowly sinking into the ocean. Will any of them manage to escape?
We open with a montage of drawings depicting underwater scenes and the credits. A launch from a seaplane approaches a beach on a Pacific Island. The first to step ashore is Dr. Karl Weigand (Leslie Bradley) the leader of the second expeditionary team to visit the island. Next is Dr. James Carson (Richard Cutting), he is the geologist. Sporting the French accent is Jules Deveroux (Mel Welles). The only female member of the team is Martha Hunter (Pamela Duncan) and last is her fiancé, Dale Drewer (Richard Garland). The seaplane pilot, Ensign Quinlan (Ed Nelson) is anxious to leave as there is an approaching storm. He was on the island once before to rescue the first group of scientists sent to study the island. No trace of the first group was ever found. The group walks up to the house the Navy built. Quinlan supervises the second launch loaded with supplies. Seaman Tate (Charles B. Griffith) loses his footing and falls overboard. He screams underwater and by the time he is brought back on board his head is missing. Seaman Ron Fellows (Tony Miller) and Sam Sommers (Beach Dickerson) are left to assist the scientists. (Note: the characters and actors are reversed in the credits, and Sommers is identified as Jack, even though he is addressed as Sam.) They pitch a tent on the beach. They have a large supply of dynamite and hand grenades to assist the geologist with his work.Quinlan meets the scientists at the house and confirms that Tate's head was gone from his body. Hank Chapman (Russell Johnson) is a radio operator and technician. While the scientists unpack, Hank returns a box of dynamite back to the beach. Weigand questions Quinlan prior to his departure. He wants to confirm there was no trace of the first scientific group to survey the island. Only a journal was found, no other physical evidence is the reply. Quinlan speculates that they were out at sea when a hurricane swept the island; a very common occurrence.Everyone meets down on the beach to see Quinlan off. Along the way, the island is rocked by explosions and earthquakes. A rock slide comes close to the sailors. Mac (Maitland Stuart) is anxious to leave. He fires up the launch and takes Quinlan back to the seaplane. The scientists walk up to the cliffs to watch the seaplane depart. Hank explains to Fellows and Sommers what brought everyone to the island. The H-Bomb test on Bikini left a huge amount of fallout on this island. Weigand's group is here to study fallout effects. Carson is a geologist and he will be studying the soil. Deveroux is a botanist and will study the plant life for radiation poisoning. Hunter and Drewer are biologists, he concentrates on land animals; she studies sea life. Weigand is a nuclear physicist. He will collect his team's findings and relate them to current theories on effects of radiation. They wave to the departing seaplane, but just as it lifts off it explodes.Hank rushes back to the house to radio the accident. A tropical storm hits the island. The scientists decide to let Hank continue trying to contact the Navy while they start work–reviewing the journal left by the previous expedition leader, McLane. Weigand reads the journal. He covers the passage about a worm that is very large and seemingly indestructible. The journal ends abruptly, with no more detail about the giant worm-like creature. After Hank asks about the giant worm the house is rocked by an earthquake.Weigand and Dr. Dale Drewer stay up and talk. Drewer notes that he hasn't seen any insect life. They are interrupted by a ticking or crackling sound from outside. They go outside, but only discover a wire brushing against the house.The next morning, Dr. Martha Hunter puts on scuba equipment and begins her exploration of the sea. She examines some seaweed and a giant clam. Dale joins her on the bottom. They continue to explore. She collects a seaweed sample. He directs her back to the beach. They walk ashore and she complains, "You nearly frightened me to death. I was scared, and lost too. I was using a large black rock as a landmark, but when I returned it was gone." Weigand and Carson call to the couple on the beach. They are told to take the path. They warn them about recent geological activity that opened a large pit along the path. Carson wants to explore, but Weigand forbids it. Carson notes that the rocks are unusual, like they were fired in a kiln.That night Martha is awakened from a sound sleep by the voice of McLane. He calls to her specifically and by name and beckons her to come to him. She dresses and goes outside. She again hears his voice and pleas for help. She is startled by Dr. Jim Carson outside too. He tells her he heard the same voice call to him. The two walk to the pit. Carson ropes up and repels down into the pit. Another earthquake strikes and Martha falls over striking her head on a pick axe. We hear Carson scream. The rest of the team arrives and asks about Dr. Carson. Martha tells them, "He's in the pit; I saw the rope go slack." They call him and he replies, "My leg, it is broken."Weigand suggests a rescue attempt via the beach and cave system. The rest feel he is hiding something from them. The two sailors join the rescue mission and report that whole sections of the island are crashing into the ocean. They enter the cave system.Dale and Martha are in the house, reviewing the journal about the cave formations only happening at night. They hear the boom of an earthquake and a curious ticking sound. Dale grabs a pistol and they go to the front door. They hear the sound of wood splintering from a room in the house. As Dale enters, a giant crab claw strikes out at him. Disarmed of his gun, he retreats back to the living room.In the caves, Weigand, Deveroux, Hank and the two sailors call out to Dr. Carson. He replies and asks them to come quickly.In the house, Dale and Martha are left in the dark when a fuse pops and they hear a roar. The ticking sound fades.The rescue party arrives at the place where Carson was lost. The rope is still hanging from above the pit opening. They discover blood on the cave floor. Not finding Carson they decide to return in the morning. Weigand insists they exit the cave via the rope. When the sailors object, as their tent is on the beach and closer using the cave, Weigand orders them up the rope.Dale enters the room to find it a shambles. The radio has been damaged and the lab rats missing. The next morning the scientists examine the room more carefully. There is a hole in the wall to the outside. Deveroux asks Hank if he can fix the radio; Hank isn't sure. Dale asks Weigand if electricity may be a defense. Martha looking outside notices an entire mountain is now gone. They leave the house for another rescue attempt for Dr. Carson. Another earthquake rattles the cave and they try to seek shelter against the cave walls. Deveroux falls and a large boulder severs his hand at the wrist. They hastily improvise a tourniquet as Fellows and Sommers enter the cave. They report that most of the island has now fallen into the ocean. They also report having heard Carson's voice.In the house, Martha gives Deveroux a shot to help him sleep. He mumbles in French as she wipes his brow; He falls off to sleep. On the beach, the two sailors play cards using sticks of dynamite as currency. They hear a peculiar ticking sound getting louder. Fellows describes it as the sound of "a kid dragging a stick over a picket fence." Sommers grabs the kerosene lamp to explore and comes face to face with it. He screams and falls back, collapsing the tent.Deveroux is awakened by the voices of the two sailors. They tell him they found Dr. Carson. They tell him to come to the pit. When he exits the house and gets near the pit he is attacked by a giant crab claw. It latches on to his neck. Martha is awakened by his screams. The rest join her in the living room. They hear the voice of Deveroux from his room, despite it being empty. Weigand notices the voice is coming from a metal ash tray on the nightstand next to the bed. Deveroux promises to return the next night. They speculate on how Deveroux can be dead and why they still hear him.The remaining party meets on the beach. They examine the remains of the sailor's tent and notice most of the dynamite is missing. Hank gathers up the remaining pieces and returns to the house to work on rebuilding the radio. That evening, Deveroux talks to them via a pistol. When they ask about Carson, Carson replies in his voice. The two voices tell them to come to the pit. Weigand, Hank and Dale go to the pit. They hear the ticking and are attacked by a giant land crab. It approaches and is impervious to bullets and hand grenades. But a stalactite that is dislodged from the cave ceiling imbeds itself in the crabs brain and it stops. They are not sure if it is dead. Weigand separates the claw from its arm and they take the specimen back to the house. They hear a second crab approaching. They photograph both and set a charge of dynamite to destroy the cave. The voice of Deveroux tells them they have destroyed McLane and all of his party.As Hank works on the radio, Weigand performs tests on the claw. He concludes radiation poisoning caused a freak. It is explained that since electricity is the flow of free electrons, the crab is composed of free atoms. Like a liquid with a permanent shape. The crab eats the brains of its victims and absorbs their minds and memories. Martha, looking at the photographs, notices that one of the crabs is about to reproduce. Hank hooks the claw up to a battery and it glows, and then is reduced to ashes. They create an electrical device to destroy the crab. They place it in the cave. Weigand's plan is to disable the crab so they can dissect and study it. The earthquakes continue. Hank and Martha enter the cave and set the devices. The crab chases the two out of the cave and into the ocean. They swim for shore with the crab right behind them. The crab comes ashore. Dale shoots it with his rifle but that only annoys it. The crab continues to destroy the island leaving the humans a smaller space to occupy. They return to the house. Hank tries to get someone on the radio. He picks up a station from Hawaii, but tells Martha only the receiver works, not the transmitter. Weigand and Dale Drewer explore what little island remains. Weigand finds oil and wants to trace the origin. He traces it to the cave and trips the electronic trap set earlier. He is paralyzed. The crab attacks and consumes him. Dale, Martha, and Hank witness his death and exit the cave.They return to the house and send an S.O.S. with a telegraph key, but the crab blocks the transmission. The island is struck by another earthquake that destroys the house. They make their way to a rocky outcropping where their transmitter antenna is located. The crab walks towards them. They try explosives but they are of no use. Dale fights it off with a hatchet, but Hank has a better idea. He pulls the transmitter on to the crab and electrocutes the creature. We close with Dale and Martha embracing.