- Directors: Eric Leighton, Ralph Zondag
- Producers: Baker Bloodworth, Pam Marsden
- Writers: John Harrison, Robert Nelson Jacobs, Walon Green, Thom Enriquez
- Genres: Animation, Adventure, Family, Thriller
- Actors: D B Sweeney, Alfre Woodard, Ossie Davis, Max Casella, Hayden Panettiere
A mother Iguanodon watches over her nest. A young Parasaurolophus wanders up and looks into the nest, but the mother shoos it away. When the young reptile disturbs a large Carnotaurus hiding in a nearby forest, the whole herd of dinosaurs is sent running away from it, Gallimimus, Edmontosaurus, Pachyrhinosaurus and such. One Pachyrhinosaurus is unable to escape and is devoured. During the chaos, the Carnotaurus tramples most of the Iguanodon’s eggs, leaving only one still intact. The surviving egg, stolen by a number of other dinosaurs, eventually ends up on Lemur Island, an island inhabited by, as its name suggests, lemurs and a gigantic flightless bird the Gastornis. A baby Iguanodon soon emerges from the egg, to the fear of some and the delight of others among the tiny mammals. The lemurs are initially cautious about the dinosaur, but they nonetheless raise him, giving him the name Aladar. Not long after Aladar has grown up, a tremendous meteoroid falls from the sky during a Lemur mating ceremony, landing in the ocean. The resulting fiery shockwave destroys Lemur Island and kills the Gastornis. Aladar escapes with Plio, his mother-figure and mother of Aladar’s adoptive siblings, Zini and Suri, and his grandfather-figure, Yar, by jumping into the ocean. They wash up on shore to find that they are supposedly the last of the lemurs.
Suddenly, the surviving Carnotaur comes round the rocks and corners the herd. The hungry carnivore charges, but is discouraged from a meal when the entire herd (led by Aladar) stays together as opposed to scattering like it hoped. The herd puts up loud calls at the Carnotaur without any act of cowardice, forcing it into a corner to let the herd pass away from it for good. The Carnotaur was about to give up on its hunt until it spots Kron climbing the wall, and moves to attack him. Neera and Aladar follow to save Kron. Upon reaching the top of the rocks and spotting the sheer drop on the other side, Kron realizes he is now cornered by the predator. Kron charges into battle; for a moment he manages to hold his, but in the last minute, he is bitten by the Carnotaur and thrown hard against the rocks. The Carnotaur closes in to kill Kron, but is stalled as Neera and Aladar come to the cliffside. In the fight that follows, the Carnotaur gains the upper hand by knocking Neera and Aladar out of its way, but Aladar gets back up and knocks it to the edge of a cliff, which starts to break as being unable to hold the theropod’s heavy weight and the Carnotaur plummets down the cliff. The Carnotaur desperately tries to drag Aladar with it, but instead leaves Aladar with a bite on his back, and Aladar saves himself by holding on to what’s left of the edge of the cliff while the Carnotaur falls down on the rocky grounds to its death. Neera goes to Kron’s aid but it is too late, as Kron has already died from his wounds. Aladar takes over as the new leader and leads the herd through the cave into the nesting grounds, where the lemurs also find that they are not the last of their kind, and that there are lemurs in the nesting grounds. Zini finally gets a mate, Suri gets new friends and the lemurs have a new tree like the old one. This one has a lake for the lemurs to drink and swim. The movie ends with one of Neera and Aladar’s eggs hatching, showing the lemurs and dinosaurs’ peaceful life in the valley. Plio’s voice can be heard above the jubilant bellows of nesting mothers, saying, “None of us really know what changes, big or small, lie ahead. One thing is certain; our journey is not over. We can only hope that, in some small way, our time here will be remembered.”