Anthropology professor George Challenger (Claude Rains), explorer Lord John Roxton (Michael Rennie) and an assorted team of thrill seekers and experts trek through a Venezuelan rainforest that they suspect may be home to living prehistoric creatures. Before long Challenger, Lord John and company discover a valley filled with vicious dinosaurs and oversize arachnids — unfortunately, their chopper breaks down, leaving them stranded in one of the deadliest environments on Earth.