In the late Cretaceous period of Earth's history, iguanodon Aladar (voiced by D.B. Sweeney) is separated from his own species while still inside his egg. He is taken in and brought up by lemurs Zini and Plio, but when this adoptive family is all by wiped out by a meteor shower, Aladar and his friends are forced to join a mixed herd of dinosaurs who are migrating to a new nesting ground.

This tribe is led by the hard-headed Kron (Samuel E. Wright), whose 'survival of the fittest' approach clashes with Aladar's more altruistic nature.