When the Pteranodon family takes the Dinosaur Train under the sea to visit their friend Elmer Elasmosaurus, they learn that the Elasmosaurus family is going away from the station to follow their food, the fish. Buddy wants to follow, but the train tunnel doesn't go that way - so the Conductor decides to take the family in a brand-new invention, the Dinosaur Train Submarine! On their journey, they meet a marine reptile called Otto Opthalmosaurus, who guides the submarine deeper than it's ever been, and even helps them find their way out of a sea cave. During a day at Troodon Town Music Festival, the Pteranodon family and King Cryolophosaurus hear a wonderful singer named Crystal Cryolophosaurus. King is very interested in becoming friends with Crystal, another dinosaur of his species, but he gets very nervous when trying to talk to her. Tiny and Buddy help King overcome his nervousness, and he and Crystal end up friends who happily sing together.