When Buddy and his family want to follow some friends on an undersea fishing trip, the conductor unveils a new invention - the Dinosaur Train Submarine! On its maiden voyage, Otto Ophthalmosaurus guides the sub deep into the ocean using his big eyes that can see in the dark. Then, the family meets Shoshana Shonisaurus, an enormous marine reptile that's something between a dolphin and a whale, and Maisie Mosasaurus, a huge fast swimming lizard with flippers. Finally, the family watches small eggs hatch on the beach. Baby Archelon turtles emerge and start to swim away. The family follows them on their journey across the ocean where they will grow up to be giant-sized sea turtles!