One day, a cloud of "Swarming Leaf Gobblers" descends upon the Great Valley and devours every plant in sight. Now the once lush landscape is barren and empty, and the Valley herds must search for a new home until the leaves grow back. But when quarrelling among the adults threatens to separate them, Littlefoot and his prehistoric pals set off on their own.

Their search takes them across the Big Water - where strange creatures live and swim - to the shores of a mysterious island where they become reacquainted with Chomper, the adorable young sharptooth from The Great Valley Adventure! This time, the tables have turned and Chomper must protect his plant-eating friends from the sharpteeth of the island - including his own parents!