Littlefoot, a young Apatosaurus living in the verdant Great Valley, is eagerly awaiting the arrival of his father Bron, who returns to the valley with his herd once a year. However, he soon learns from a ''Nothronychus'' member of his dad's troupe named Wild Arms that his father became stranded in the wilderness when the ''fire mountain'' erupted, leaving the rest of the herd to go on without him. They ask Wild Arms for help, but he belligerently says no. The other grownups agree. After worrying about his father, Littlefoot sets out alone. However, the others soon catch up with him. They do fine at first, but run into the ''earth divide''. It is suggested that they follow the ''Long Valley'' instead, but Littlefoot says that that will take too long, and that they must cross there. Luckily, they are able to knock down a tree, and get across.