The Pteranodon family goes back underwater in the Dinosaur Train Submarine and meet Maisie and Marvin Mosasaurus, a daughter and father who are huge, fast-swimming lizards with flippers. Maisie and Shiny bond when they discover neither likes to go down deep in the water. The Pteranodons and Mosasaurus have a great time up near the ocean's surface, each family showing the other how they hunt and catch fish to eat. When Mrs. Pteranodon announces that she and Mr. Pteranodon are going on a special anniversary date that night, and the Pteranodon kids will have their first babysitter ever, the kids are not very happy. The babysitter is Keira Chirostenotes, a responsible, patient teenager. Buddy, Tiny, Shiny, and Don each take almost the whole evening to warm up to her. While Mom and Dad are on their date on the Dinosaur Night Train, Mom is relaxed, but Dad really misses the kids and can't stop thinking about them. In the end, Mom and Dad rush back to the nest to find all the kids and Keira having a great time!