When heavy rains create a mysterious "new water", Littlefoot sets off to explore the Great Valley. He quickly becomes friends with Mo the mischievous Ophthalmosaurus who has been isolated from his pod by the weather. When Littlefoot and friends get separated from their parents because of the Earthshake, they help Mo get back home to the Big Water, while avoiding a hungry pliosaur. On the way, Littlefoot and Mo discuss such interesting and see dangerous things like imaginary friends, the pliosaur, the concept of loneliness, and the true meaning of a brother.