In prehistoric times, a severe drought causes “leaf-eater” dinosaurs to leave their homes to search for the mythical “Great Valley.” As different leaf-eaters herds migrate, the last surviving “longneck” egg hatches. Its mother names him Littlefoot, and introduces him to his grandparents. Later, Littlefoot’s mother finds a tree with a lone “tree-star.” Plucking it from a branch, she tells Littlefoot the Great Valley is full of tree-stars. Before he eats the tree-star, Littlefoot hears laughter and sees Cera, a young “three-horn” dinosaur, playing. When he wanders over to her, Cera’s father, Daddy Topps, intervenes. Daddy Topps informs Cera that three-horns do not associate with longnecks. Afterward, Littlefoot asks his mother why he is not allowed to be friends with Cera. She tells him that leaf-eaters have always stayed within their own herds, and promises there are other young longnecks in the Great Valley for him to play with. Littlefoot wishes they were already at the Great Valley, but his mother explains they must continue to travel: by following the “bright circle” to a large rock resembling a longneck, and then through the “mountains that burn.” Later that night, Littlefoot chases after a “hopper.” As he follows the hopper away from his family and into a pond, he runs into Cera. Finding more hoppers in the water, they work together trying to catch them. While playing, a fierce dinosaur, “Sharptooth,” attacks. Littlefoot’s mother arrives and fights Sharptooth to rescue Littlefoot and Cera. However, Sharptooth bites her on the back. While Littlefoot, his mother and Cera flee, a powerful “earthshake” occurs, breaking the ground apart and creating a wide chasm. Sharptooth falls in. When the shaking stops, Cera is separated from her parents on the opposite side of the chasm, as is Littlefoot from his grandparents. Littlefoot finds his mother dying from Sharptooth’s attack. She asks her son if he remembers her directions to the Great Valley. After he says yes, Littlefoot’s mother tells him she will always be with him, and dies. Heartbroken, Littlefoot meets an older leaf-eater named Rooter, who tells him it is not his fault, nor his mother’s fault, for dying. As Littlefoot cries, Rooter leaves. Later, Littlefoot sees a tree-star with dew on the ground. Looking at it, he hears his mother’s instructions on getting to the Great Valley. Heading back to the chasm, Littlefoot sees Cera looking for a way to the other side. He invites her to join him on his journey, but Cera refuses. As she wanders down into the chasm alone, Littlefoot leaves. Later, he finds a pond and meets Ducky, a happy “swimmer.” Still upset about Cera not wanting to travel with him, Littlefoot informs Ducky that longnecks do not speak to swimmers. However, Ducky insists she must be a longneck as she has a long neck and tail just like Littlefoot. When Littlefoot does not respond, Ducky admits she is more of a “bigmouth,” and was separated from her family during the earthshake. Taking a liking to Ducky’s cheery disposition, Littlefoot asks her to join him. Later, Littlefoot and Ducky become hungry. Stopping at a palm tree, Littlefoot grabs a palm frond with his mouth, and Petrie, a small “flyer,” falls out. Petrie tells them it took him a long time to climb the tree. When Littlefoot and Ducky ask why he did not fly up into the tree, Petrie admits he is afraid to fly. Littlefoot and Ducky invite Petrie along, and the three new friends continue on their way to the Great Valley. While walking inside the chasm, Cera persists in finding a way to the other side, and finds Sharptooth unconscious on the ground. Wanting to prove herself a brave three-horn, Cera repeatedly rams her head into Sharptooth’s head. However, Sharptooth is awakened by her actions and begins to move. Scared, Cera runs out of the chasm. Later, she catches up with Littlefoot, Ducky and Petrie. Cera lies and tells the group she fought Sharptooth by herself. Littlefoot says Sharptooth is dead, but Cera insists she defeated him, and demonstrates her fighting skills. As she reenacts her battle, she accidentally launches Ducky into the air. Upon landing, Ducky finds a “spiketail” hatching from an egg. Ducky names the young spiketail Spike, and invites him to join her friends as they search for the Great Valley. Later, the group finds a small stand of trees. Suddenly, a herd of fast-moving leaf-eaters run past and devour the majority of tree-stars. After the herd departs, Littlefoot, Ducky, Petrie, Spike and Cera find one small tree with tree-stars on the high branches. Being too scared to fly, Spike, Littlefoot and Ducky lift Petrie to reach the tree-stars. After pulling down enough to share, Petrie loses his balance and falls. However, his friends encourage him to spread his wings, allowing him to glide safely down. After eating, they huddle together and fall asleep. In the morning, Sharptooth awakens the group. Escaping through a small crack in a rock wall, Littlefoot sees the rock formation in the shape of a longneck. Encouraged, he leads his friends toward the mountains that burn. Later, tired of climbing, Cera decides to find an easier way to the Great Valley. Littlefoot attempts to explain his mother’s directions, but Cera responds that his mother was wrong. Angry at Cera’s comment, Littlefoot fights her. After the fight, Cera leaves. Ducky, Spike, and Petrie choose to follow after Cera in search of another way. Disappointed, Littlefoot continues alone. While following Cera through more burning mountains, Petrie falls into a tar pit. When he calls out for help, Cera ignores him. Ducky and Spike attempt to save Petrie, but they become trapped on a rock in the middle of lava. Littlefoot appears and rescues Spike and Ducky first. Afterward, they try to save Petrie from the tar pit, but fall in. Elsewhere in a cave, three dinosaurs with hardheads surround Cera. However, a tar monster appears and scares off the hardheads, leaving a frightened Cera. As the monster approaches, Cera sees it is Littlefoot, Spike, Ducky and Petrie covered in tar. Embarrassed by appearing weak and getting lost, Cera hides from them and cries. After cleaning off the tar in a nearby pond, Littlefoot sees Sharptooth in the distance, and comes up with a plan to stop Sharptooth: by pushing a boulder onto him, and knocking him into the pond’s deep trench. Ducky is nominated to lure Sharptooth into the water. Ducky gets Sharptooth's attention, and he chases her into the pond. On a cliff above, Petrie signals Spike and Littlefoot to move a boulder, but they struggle to move the rock. Sharptooth sees them and rams his head into the cliff, knocking Petrie over the ledge. While Petrie falls, the air puffed out from Sharptooth’s nostrils gives him a boost. Flapping his wings, Petrie realizes he is flying. Seeing Ducky still in trouble, Petrie tries to distract Sharptooth. Angry, Sharptooth leaps onto the boulder, preventing Littlefoot and Spike from moving it. Cera appears and rams her head into the boulder, causing Sharptooth to fall. While falling, Sharptooth grabs Petrie and the two disappear under the water. Ducky, Cera, Spike and Littlefoot cry for their lost friend, but Petrie climbs out of the water alive. Afterward, Littlefoot hears his mother’s voice and sees her form in the clouds. As the clouds move, Littlefoot runs after them, and sees the Great Valley. In the valley, Ducky finds her parents and siblings. She introduces Spike as part of the family, even though he is a spiketail. Petrie locates his mother and siblings and shows them he can fly without fear. Cera also finds her father, Daddy Tops, and Littlefoot happily reunites with his grandparents. Afterwards, Littlefoot, Cera, Ducky, Petrie and Spike meet up to play together as friends.