In the Late Cretaceous Period, a female Maiasaura finds an egg and adopts it as her own. She takes it to her nest near Egg Mountain, a volcano with a giant egg shaped rock. One day, a marauding pack of Troodon attack the nesting grounds, snatching and eating the Maiasaura eggs. The mother loses all but two of her eggs, one of which being the egg she saved. When the eggs hatch, one is a male Maiasaura, whom the mother names Light, and the other is a Tyrannosaurus, named Heart. The herd leader plans to kill Heart in order to prevent him from eating anyone when he gets older, but the mother Maiasaura stops him, arguing that he is a newborn and harmless. The leader gives her the chance to leave him in the wild, but she feels guilty about abandoning Heart and decides to leave the herd and raise her children by herself.