Jamie and Tom are back in the Permian era and decide to do some snorkelling and look at all the weird and wonderful creatures beneath the sea. Wanna can't swim but he wants to come too, so the boys make him a boat from an enormous trilobite shell. The three friends discover a secret pool of water and decide to explore ...but disaster strikes! They're not the only ones in the pool. They've been joined by three prehistoric sharks with ferocious unravelling jaws full of razor-sharp teeth. When Wanna falls off his shell-boat, it's up to Jamie and Tom to lure the sharks away before their little dino-friend becomes Permian shark food!