After a swarm of "leaf-gobblers" descends upon Great Valley, devouring all plants and leaving it a barren wasteland, the inhabitants must seek another place where they can survive until the plants in Great Valley have grown back. However, the Leaf Gobblers have left a path of devastation behind them, leaving no food for the dinosaurs to find. After searching for many days, tension enters the group as Cera's father and Littlefoot's grandfather argue over changing their course, which sparks a fight between Littlefoot and Cera. The fight soon breaks up when Cera's father announces that each herd will go its own way the next morning. Not wishing to be separated, Littlefoot leads his friends off in the night, in the hope they can find food before they are caught. They leave a trail for the grownups to follow, and eventually reach the "Big Water". Although they are disheartened by the undrinkable water, they spy a verdant island connected to the mainland by a thin land bridge. On their way, a tsunami occurs, which they narrowly escape. They quickly tuck in to the plentiful bounty of the island, but are then horrified to discover the tsunami has destroyed the land bridge.
Most of the plot is unknown. The Ghost of Slumber Mountain originally took up 3000 feet of film and three reels, equivalent to approximately 40 minutes. After the film premiered at the Strand Theater, manager Walter Hayes ordered Dawley to cut the film down to about one reel because it was too long. A restored version runs approximately 19 minutes.
The film opens with a trio of young bluebirds harassing their youngest sibling, Buster (Blaze Berdahl). As Buster leaves his family, he meets an intelligent orange T. rex named Rex (John Goodman) playing golf. He explains to Buster that he was once a ravaging dinosaur, and proceeds to tell the story of how he came to become what he is today.
Littlefoot and his friends have recently arrived in the Great Valley. The children are living happily in their new home, under their families' watchful eyes. When the gang tries to get to the sheltering grass to play, but land in the sinking sand surrounding it, the grownups come and save them and chastise them for disobeying the rules: "don't cross the sinking sand without help." Cera then tells the others that they need to prove they are independent by running away for a while into the Mysterious Beyond, a location just outside the valley. Before they leave, they notice two egg nappers stealing an egg from Ducky's nest. They chase them into the Beyond, during which a landslide occurs. The egg rolls safely back to the nest, but the five are under the impression that it was destroyed in the landslide. This upsets Ducky until she notices what looks like the egg, only slightly larger.
Littlefoot's grandfather one night tells the children a story about "The Lone Dinosaur" (a pun on The Lone Ranger), a legendary longneck who once protected the Great Valley from the most ferocious sharptooth ever to live. A fight ensued, which led to the Sharptooth's death. However, the sharptooth left "The Lone Dinosaur" with a scar slashed across his right eye. Soon after the battle, a huge monolith that resembled a proud sauropod, having life-sized Sharptooth teeth arranged around his neck, came out of the ground during an earthshake. The dinosaurs called it "Saurus Rock". The legend also states that if anyone damages the monolith, bad luck would descend upon the valley.
Tourists Ben Hildebrand and Eric Kirby decide to parasail around the island of Isla Sorna. A creature of unknown species attacks and kills the boat crew as it passes through a fog bank, but Ben uncouples the line, and with Eric bound with him, he goes sailing into the wilderness.
Escaping from China with a microfilm of the formula for the mysterious "Lotus X", Lord Southmere, a Queen's Messenger, is chased by a group of Chinese spies.
The year is 196303 BC. A group of cavemen rises from the mud, and the first thing on their minds is finding something to eat. After two unsuccessful attempts, the group decides to eat one of their own. Suddenly, the men meet women, but don't know how to make love to them. The elder of the men sees a pair of stegosaurus performing doggystyle, which inspires the elder to do the same thing with one of the women. Nine months later, that same woman is pregnant with two boys, Ah and O. The cavemen like Ah, but are frightened by O and abandon him. Meanwhile, a brontosaurus abandons one of her eggs. Both O and the baby brontosaurus Igua meet, and instantly become friends.
After a period of harsh rain, the Great Valley is partially flooded and covered in debris. Littlefoot rushes to find his friends after the storm ends, but they are preoccupied: Cera and her father are removing a large log from their nesting area, Ducky and Spike are relocating their nest with their family, and Petrie has gotten a cold. Littlefoot is bored and instead of helping his friends clean up the dead bodies he selfishly wishes for a brother (a wish that will be granted in the next movie, although ironically they will not get along). Eventually he explores on his own and discovers a large area flooded by the rains. The adults advise their children to stay away, fearing creatures from the outside may have been washed into the Valley. However, Littlefoot thinks he knows best so, ignoring the advice, he goes anyways and meets Mo, a playful young "swimmer" that had been caught in the floodwater.
Godzilla cells brought into space by Biollante and Mothra are exposed to intense radiation from a black hole. The celestial fission creates a highly aggressive extraterrestrial beast closely resembling Godzilla. This "SpaceGodzilla" quickly makes its way to Earth.
Ducky is upset with her brother Spike, for keeping her awake at night with his snoring (called "sleep rumbling" by the characters), and for eating her treestars. She tells Cera about her feelings, and confides that she does not know how to express them, even though they have lived together for eight movies, to which Cera agrees to teach her. Yes, that's right. Angry Cera who storms off like every other movie is going to be the emotional guide. At the same time, the children are attending an unaccredited school of sorts, in which they are taught by an old Pachyrhinosaurus named Mr. Thicknose, who falsely professes to have been everywhere and seen everything. Littlefoot is eagerly questioning all the topics they cover, to the old dinosaur's irritation, and blows the top when he startles Mr. Thicknose into running into a tree, upon which a melon falls on his head. Mr. Thicknose complains to Grandpa Longneck about Littlefoot's behavior, and Grandpa Longneck, knowing Littlefoot meant no harm, gently explains to him why he must be more respectful to Mr. Thicknose.
Littlefoot notices a longneck herd entering Great Valley, and informs his grandparents. They tell him that the longnecks are their cousins, and that they must go and greet them. As the Old One, the leader of the herd, tells the residents of the great valley that her herd has been migratory since a heavy rain period turned their old home into a marshland called "The Land of Mists", and became the home of many dangerous creatures, Littlefoot meets a longneck girl named Ali, and invites her to play. He introduces her to his friends, but as she is not used to associating with diverse species, she is afraid of them.
Atouk (Starr) is a bullied and scrawny caveman living in "One Zillion BC – October 9th". He lusts after the beautiful but shallow Lana (Bach), who is the mate of Tonda (Matuszak), their tribe's physically imposing bullying leader. After being banished along with his friend Lar (Quaid), Atouk falls in with a band of assorted misfits, among them the comely Tala (Long) and the elderly blind man Gog (Gilford). The group has ongoing encounters with hungry dinosaurs, and rescues Lar from a "nearby ice age", where they encounter an abominable snowman. In the course of these adventures they discover sedative drugs, fire, invent cooking, music, weapons, and learn how to walk fully upright. Atouk uses these advancements to lead an attack on Tonda, overthrowing him and becoming the tribe's new leader. He rejects Lana and takes Tala as his mate, and they live happily ever after.
In the opening scene, Max Anderson (Jean-Michel Paré) is being pursued by a Giganotosaurus. Max then comes across a fissure vent. When he tries to jump across, he falls while calling out the name of his brother. Ten years later, Max's brother volcanologist Trevor Anderson (Brendan Fraser) is visited by Max's son/Trevor's 13-year-old nephew Sean Anderson (Josh Hutcherson). In a box of items that belonged to Max is a book, Journey to the Center of the Earth by Jules Verne. Notes written by Max are found inside the book.
Les paléontologues du Musée américain d'histoire naturelle partent à la rencontrent des dinosaures du Triassique, du Jurassique et du Crétacé, du désert de Gobi en Mongolie aux spectaculaires paysages de grès du Nouveau Mexique. La magie de l'image numérique permet aux spectateurs de voir, à partir de ces récentes découvertes, des dinosaures... vivants !