A Longneck (Brontosaurus) herd gives birth to a single baby, named Littlefoot. Littlefoot comes across Cera, a Three-horn (Triceratops), practicing head-butting while trying to smash a Beetle with her horn; Littlefoot thinks it is funny. Cera challenges Littlefoot, who accepts and the two children charge headfirst toward each other until Cera’s father, Topsy, intervenes. Littlefoot’s mother (Helen Shaver) explains the different kinds of dinosaurs: Three-horns, Spiketails, Swimmers, Flyers and that they’re different and it’s always been that way, but she promises Littlefoot that there will be other longnecks for him to play with in the great valley. That night, as Littlefoot follows a Hopper (Frog), Cera plays with Littlefoot for a while until a Sharptooth (Tyrannosaurus Rex) attacks. During their escape, Littlefoot accidentally blinds Sharptooth in one eye with a thorn. Littlefoot’s mother battles with Sharptooth and suffers severe injuries, but defeats him by pushing him into a deep chasm.
At the same time, an earthshake (earthquake) occurs, opening a deep ravine in the ground. It swallows up the Sharptooth and many other dinosaurs, separating many herds, including Littlefoot’s and Cera’s. Littlefoot finds his mother when the earthquake ends, but she is near death. She gives Littlefoot instructions on how to get to the Great Valley and how to follow the bright circle (sun) past the great rock that looks like a longneck and then past the mountains that burn (volcano). Now feeling depressed and confused, Littlefoot by chance slides down a ravine, where he meets an old (Polacanthus) named Rooter, who consoles him upon learning of his mother’s death. After mourning for some time, Littlefoot begins to search for the Great Valley, and tries to get Cera to join him, but she refuses.
Littlefoot meets a Bigmouth (Saurolophus) named Ducky, who asks to join him. As they try to find food, they encounter an aerophobic Flyer (Pteranodon) named Petrie, who joins them on their quest. Cera finds Sharptooth’s body. It is not dead, only unconscious, causing Cera to flee in terror. Cera later bumps into Littlefoot, Ducky, and Petrie and warns them of Sharptooth, but Littlefoot is convinced that Sharptooth is dead. As Cera describes the encounter (exaggerating her bravery), she accidentally flings Ducky near a patch of grass, which has a hatching egg containing a baby “Spiketail” (Stegosaurus). Ducky names him Spike and brings him into their group. Seeking the Great Valley, they find a river leading to a cluster of trees, which is abruptly depleted by a herd of starving sauropods. Searching for remaining growth, they discover a single leaf-bearing tree, and obtain food by piling up ontop of each other and pulling it down. Cera remains aloof and arrogant, boasting that she is independent, but at nightfall, she, along with the others, gravitates to Littlefoot’s sleeping nest for warmth and companionship.