A group of scientists is taken to a secluded island where nuclear weapons testing occurred in 1958, in order to investigate sightings of mutated animals, and allegations of a monster. On the island, overgrown crabs are discovered, and the researchers and their ship's crew are stalked and spied on by an unseen being. After having sex with his wife, Dr Simone, Professor Keller goes for a swim, and is drowned by the monster, which then rapes and kills his wife. At the main camp, the ship's two crew members, Pierre and Jacques, go off to collect fruit, and are followed by the beast. Pierre is bludgeoned with a rock, while Jacques is beaten to death with a log. Hearing the two men being attacked, the rest of the expedition goes to investigate. The monster captures Annie and takes her to its cave lair, where she finds a journal. The book reveals the creature was once Antoine Domoduro, one of the island's original inhabitants. He became stranded there with his wife and daughter during atomic bomb tests, which killed his family and mutated him. The next day, the others discover the bodies of their companions, and that the boat is gone. Captain Hardy goes in search of Annie, and finds Benoit, a reporter who had followed the researchers to the island. Dying due to severe injuries, Benoit tells Hardy that he was attacked by an "ape man" that is holding Annie captive in a nearby cave. Back at the camp, Antoine strangles Professor Lemuan, and kills then rapes Countess de Saint Jacques, leaving Hardy to try and rescue Annie on his own. Hardy finds the cave and frees Annie, but while fleeing the two are confronted by Antoine, whom Hardy shoots in the chest with a speargun. Antoine rips the spear out and tries to attack Hardy with it, but is distracted when Annie calls out his name. Antoine falls to the ground dead, and Hardy and Annie leave the island on Benoit's boat, on which the two celebrate their escape by having sex.