The story begins as we see a mother iguanodon dinosaur protecting her nest of eggs.
Suddenly a Carnotaur comes out of the forest and destroys the nest and chases all the dinosaurs away. One egg survives and it is stolen by first another small dino, then dropped in a river only to be snatched up by a bird then finally dropped into another forest far away.
The egg is found by a family of lemurs and after hatching, is raised as one of their own.
One night hundreds of tiny asteroids hit the earth . This causes a great disaster and Aladar, the now grown up dinosaur, with the lemur family with him, run for their lives. The next morning they discover that the earth is now a great wasteland.
They meet up with another herd of assorted dinosaurs on a journey to find the nesting ground. The group is lead by Kron, a hard driving iguanodon who has a sister named Neera. Along the way, Aladon, the lemurs and a few misfit dinosaurs separate from the pack and after fighting off two Carnotaurs, they find the nesting ground.
Aladar realizes that the path that Kron is taking the rest of the herd is doomed for failure so he rushes back to show them the correct way. Kron won't accept the fact that Aladar is right and won't go. He gets real upset when his sister and the rest of the herd begin to follow Aladar.
A Carnotaur appears and in a great battle, Kron and the Carnotaur are killed.
Aladar leads the rest of the herd to the nesting ground.
At the end of the film, he and Neera have their own eggs to watch over as do many of the other dinosaurs.
Spoiled by: The Movie Spoiler