A Carnotaurus ambushes and pursues an infant Parasaurolophus, triggering a stampede and forcing an Iguanodon mother to abandon her nest. The Carnotaurus kills a Pachyrhinosaurus and most of the eggs in the nest are destroyed or eaten. The one surviving egg journeys through several predicaments, including being carried off via the flight of a Geosternbergia, before ending up on a far away island populated by lemurs. Plio names the hatched baby Aladar and raises him, despite Yar's objections. Years later, Aladar and the lemurs take part in a mating ritual, where Zini does not have a mate. Moments after the ritual ends, a giant meteor strikes and destroys the island, killing the rest of the lemurs. Aladar, Plio, Zini, Yar and Suri jump into the sea, and flee to the mainland. They mourn for their losses before moving on. While crossing the deserted wastelands, they are ambushed by a group of Velociraptors. After escaping, the family comes across a remaining group of dinosaur led by Kron and Bruton, who are on a journey to reach the "Nesting Grounds", a valley said to be untouched by the meteor. While Aladar befriends Baylene, Eema and Url, Kron permits them to follow the herd. Together, they migrate across the desert to a lake they have relied on for past trips. It has seemingly dried up and Kron orders the herd to move on. However, Aladar and Baylene discover the buried water under the surface, in order to save the herd from dehydration. Impressed by Aladar's compassionate ways, Neera begins to fall in love with him. Meanwhile, two Carnotaurus follow the herd's trail and begin stalking them for food. They later ambush Bruton and an Iguanodon scout whom Kron sent ahead. While the Carnotaurus kill the scout, the injured Bruton manages to return to the herd and warns Kron that the Carnotaurus are following the herd, putting them in danger. Kron takes the lead and evacuates the area, leaving Aladar, the lemurs, the elder dinosaurs and Bruton behind. During a rainstorm, the stragglers take shelter in a cave to spend the night. When the Carnotaurus enter the cave, Bruton sacrifices himself by causing a cave-in to save the others, killing himself and one of the Carnotaurus. Aladar and his friends venture deeper into the cave. The other Carnotaurus survives the cave-in, leaves the cave and resumes searching for the herd. Aladar loses hope when they reach a dead end but the others convince him to keep going, relating how he inspired them to do the same. Together, they break through the dead end and find the Nesting Grounds on the other side. While exploring there, they find a large wall of rocks blocking the original entryway to the valley. Knowing that the herd will die attempting to climb over it, Aladar rushes off alone to find them, and he is pursued by the Carnotaurus. As Aladar catches up with the herd and suggests the safer way to the valley, the enraged Kron tries to kill him, but Neera stops Kron and saves Aladar. The herd decides to abandon Kron, taking Aladar as their new leader. As they prepare to leave, the Carnotaurus arrives and confronts them. Aladar rallies the herd to stand together, and they bellow their way past the predator. The Carnotaurus then pursues Kron to the edge of a cliff and injures him, but Aladar and Neera fight against the Carnotaurus, until it is knocked into the ravine, where it falls to its death. Kron dies from his injuries, and Neera and Aladar mourn for him. Aladar leads the herd to the cave as a route to the Nesting Grounds. A new generation of dinosaurs hatch sometime later, including Aladar and Neera's children. The lemurs find more of their kind, and soon they begin embarking on a new life together.
