An eccentric scientist (Claude Rains) returns from the Amazon with news of a distant plateau where creatures from the dawn of time still prowl the jungle. To prove his story, he gathers a team of explorers, including a journalist (David Hedison), a playboy-adventurer (Michael Rennie), a beautiful socialite (Jill St. John), and a pilot (Fernando Lamas) with a secret plan of revenge. But an unexpected attack on their camp leaves the group stranded in a world of dinosaurs and other exotic creatures, where humans are no longer the lords of the earth, they are helpless prey.