Tom and Jamie once again use their secret entrance to the prehistoric world to embark on another grand adventure, but instead they discover a meteorite has set the forest on fire and started a stampede that puts all their dinosaur friends in danger.

Tom and Jamie discover a prehistoric paradise and decide to explore a cave where they can swim with ichthyosaurs--dolphin-like dinosaurs--but a hungry plesiosaur adds a deadly element to their snorkeling adventure.