Bart Thompson heads an American construction company helping to modernize a small island in the Caribbean. Despite the islanders’ appreciation of the project, corrupt island manager Mike Hacker, upset that he cannot profit from Bart’s work, seeks to sabotage it. One day, just as Bart sets off a series of underwater explosions, his girl friend, Betty Piper, arrives in a motorboat. Bart motors out to warn her, but she is angered by his brusqueness and, not understanding the danger, dives into the water. She spots a monstrous being underwater and faints, but Bart has followed and is able to rescue her. Bart sends his assistant Chuck to investigate, then argues with Hacker about his attempts to hinder the project. Chuck interrupts to report the discovery of two dinosaurs preserved in ice at the bottom of the channel. Guessing that compressed gas has kept them intact for thousands of years and the explosions have blasted through the rock containing them, Bart orders them to be excavated. As the bodies are exhumed, Julio, Hacker’s young charge, identifies the first dinosaur as a Tyrannosaurus Rex, and the second as the herbivorous Brontosaurus. Bart hires local drunkard O’Leary to watch over the bodies for the evening, and while everyone retires to Chica’s restaurant, Hacker spots the frozen form of a Neanderthal man hidden in the debris. After concealing it in the bushes, Hacker plots with cohorts Jasper and Mousey secretly to transport the caveman to the mainland and sell him to a museum. Emboldened by his plan, Hacker next threatens to press false charges against Chica with the island authorities if she does not agree to be his girl friend. At the restaurant, Bart and Betty are discussing dinosaurs with Julio when Hacker enters and beginning to beat the boy. While Bart holds Hacker back, Julio flees into the jungle. Meanwhile, O’Leary spots the dinosaurs begin to stir, but assumes he is delirious from the DTs. A storm hits that cuts off all the power on the island, and within minutes, the rain melts the ice preserving the beasts, and they revive, fully functioning. While the caveman arises and runs into the jungle, the dinosaurs attack O’Leary, who manages to set off an explosion that alerts the islanders. Bart sends the islanders to the old fortress, which he plans to surround with a flaming moat. While his coworker, Dumpy, drives the heavy equipment to create the moat, Bart and Betty rush to Betty's house to use her father’s short-wave radio to call for help. Before they get there, however, the caveman stumbles into the home, scaring off Betty’s mother and destroying the ham radio. Meanwhile, Julio wanders through the jungle, alarmed to see the Tyrannosaurus eating several locals but charmed by the gentle Brontosaurus. He then enters the Pipers’ house and befriends the caveman, teaching him to eat with a fork. Hacker and his men, who have been searching for the caveman, enter the house and attack, but Julio and the caveman outwit their assailants together and escape into the jungle. By the time Bart and Betty reach the house, they spot Julio and the caveman riding away on the back of the Brontosaurus, and fearing for the boy’s life, rush after him. Betty is soon seized by the Tyrannosaurus, but the caveman spears the dinosaur’s foot until it releases her, then spirits her away into a mineshaft, pursued by Bart and Dumpy. While Betty tries to deflect the caveman’s admiration by cooking for him, Bart spots the Tyrannosaurus attack the Brontosaurus. Julio jumps off its back but joins the fray by throwing rocks at the Tyrannosaurus. Just as the fiend turns its attention to Julio, the caveman snatches the boy and brings him into the mineshaft. At the same time that the Tyrannosaurus assails the mouth of the mineshaft, Hacker and his men reach the back of the mine, but when Hacker commands them to crawl in, his henchmen flee in terror. Hacker is forced to climb into the back of the mine himself, where he holds Betty, Julio and the caveman at gunpoint. The caveman tries to defend the trio, but Hacker shoots him in the arm, then prepares to offer Julio to the Tyrannosaurus as bait. Outside, Bart is able to throw a bomb into the dinosaur’s mouth, temporarily hindering it. The animal’s thrashing causes the mine to cave in, and when Hacker flees, he is killed by falling debris. Inside, the caveman heroically holds up the roof to allow Betty and Julio to escape, only to be crushed by its weight. As Bart leads Betty and Julio to the fortress, the Brontosaurus slips into quicksand. The Tyrannosaurus turns its attention to the fortress, where they have only enough fuel to light the moat for five minutes. Bart waits until the creature is near, then sets the fire, which keeps the Tyrannosaurus at a safe distance. With only moments of safety remaining, Bart jumps into a dump truck and attacks the creature. Although the struggle seems doomed, Bart finally triumphs and knocks the dinosaur over the cliff. As the creature sinks to the bottom of the ocean, the crowd cheers. Betty and Julio embrace Bart, who explains to the boy that the caveman would never be able to survive so unfamiliar an era. As the crowd disperses, a ship full of tourists arrives at the shore, blissfully anticipating a quiet island vacation.