Tom and Jamie can't believe their luck when they find a secret entrance to a prehistoric world filled with dinosaurs! They vow to have a new adventure every day. Loaded with snorkeling gear, the boys decide to explore the cove itself. They have fun with the dolphin-like ichthyosaurs in the bay, and even tempt Wanna out into the water. But there are some not-so-friendly creatures lurking, including a hungry plesiosaur that wants a snack! Can the boys save their dinosaur friends--and themselves--from the deadliest creature in the prehistoric ocean?