Set 70 million years ago in the Cretaceous in North America, the film follows the journey of a young Edmontosaurus named Scar and his herd as they migrate from Northern Alaska to Alberta during the winter.
The film begins on a summer's day in Northern Alaska 70 million years ago and herd of Edmontosaurus and a spiky Edmontonia (identified as Ankylosaur) are feeding on the lush vegetation that grows all around them.
When night falls for the first time, Scar loses sign of the herd and spots a dark shape, which turns out to be a Gorgosaurus, which slices his face as he narrowly dodges the killer blow.
Scar finds the Edmontonia, his old friend, feasting on rotten wood and insect larvae from a fallen tree branch and tries his luck with mixed success.
The winter's approach causes the herbivores to start risking their safety and a young female Edmontosaurus gets killed by the Gorgosaurus as she wanders too far from the herd.
The next day, the Edmontosaurus and Pachyrhinosaurus herds join forces and move south for the winter towards Alberta, while the Edmontonia stays behind due to her heavy armor.
Nearby, The Gorgosaurus finds the Edmontonia outside its lair, but the bite to its leg is badly infected and it goes back into the cave to wait for easier prey to wander close.
Back in the Arctic, Patch tries to impress the female by dancing only to lose out to a more experienced male and the Edmontonia has found a last leaf.
Suddenly, the log she stands on breaks apart and the Edmontonia slides down the snow on her back and crashes into a snowdrift, utterly helpless.
He and several other Troodons attack the ankylosaur, but the Gorgosaurus, now fully healed of its injury, arrives and pulls the herbivore away from the smaller predators.
The Edmontonia is feasting on the budding greens and Patch manages to find a mate, offering scavenged meat from the dead Gorgosaurus.