One night, Grandpa Longneck tells the children stories about how their world began and about the Lone Dinosaur, a legendary Longneck who protected the Great Valley from a ferocious Sharptooth. In the tale, the Lone Dinosaur defeats the predator but bears a visible scar across his right eye. After the battle, a colossal, rock-like monolith rises from the earth during an earthquake, and the dinos name it Saurus Rock. The legend also carries a warning: if anyone damages the monolith, bad luck will descend upon the Valley.
The next day, Littlefoot falls off a cliff and is saved by a scarred Diplodocus—Doc—who seems to know the valley’s topography inside and out. This rescue sparks a spark of doubt and wonder in Littlefoot, who begins to suspect that Doc might be the mysterious Lone Dinosaur. He shares this idea with his friends and weaves the legend to back up his hunch, hoping to see Doc through a heroic lens.
Meanwhile, Dinah, the infant Dinah and Dana, the twins, sneak away to Saurus Rock, unseen by the adults. They arrive safely, but the moment of rescue becomes a frantic scramble when the twins end up on the peak of the rock. Dinah and Dana slip while trying to call for help, and in the scuffle, one of the rock’s teeth breaks off. The party continues home, but fate throws a dangerous obstacle in their path as an Allosaurus )—the living threat in the area—tracks them. To avoid the carnivore, the kids traverse a gorge on a suspended log; the log snaps under the pressure of the Sharptooth, and it tumbles away, taking the predator with it—its apparent death offering the group a brief sense of relief.
On their return, Cera’s father confronts the group, scolding Cera for not looking after the twins. He makes it clear that responsibility still rests on her shoulders, a tough lesson that lingers as the night settles in. That same night, Littlefoot has a nightmare in which he tumbles from a tooth on Saurus Rock and the monolith itself collapses. Grandpa Longneck soothes him, but the fear returns: what would happen if Saurus Rock were broken? The elder replies with a warning that echoes the legend—“bad luck would descend upon the valley”—a line Littlefoot cannot shake.
In the days that follow, misfortune seems to shadow the Valley: the watering hole dries up, a tornado rips through the land, and yet moments of courage shine through. Dinah and Dana recount how Cera saved them during the storm, prompting proud words from Cera’s father about her growing bravery. Doc steps forward to help, and Grandpa Longneck thanks him for standing by Littlefoot. But the adults still blame Doc for the valley’s string of strange events, while Littlefoot wrestles with guilt, blaming himself and his friends for the lingering misfortune tied to the broken stone.
Driven to prove Doc’s innocence, Littlefoot considers returning one of the Sharptooth’s teeth to replace the broken piece of Saurus Rock. In the process, he discovers that the Allosaurus is still alive. When danger surges again, Tyrannosaurus pressure rivals the carnivore’s, and Littlefoot is attacked. Grandpa Longneck leaps into action to shield him, only to find the Allosaurus and Tyrannosaurus briefly unite against the long-necked elder. Doc arrives to back up the friends, and the two long necks join forces to topple the rock tower, crushing the carnivores beneath. As the dust clears, an Allosaurus tooth falls from the ruins, which the children use to repair Saurus Rock.
Doc departs afterward, reminding Littlefoot that he already has a hero to depend on in Grandpa Longneck. With Doc out of sight, Littlefoot asks if the bad luck is over; the adults shake their heads, but a hopeful agreement emerges: there is no such thing as real bad luck, and there is value in taking action to protect what matters. In the closing moment, Littlefoot and Cera craft a legend of their own, recasting Grandpa Longneck as a hero called “The Brave, Great Dinosaur,” a mythic protector who inspires courage and cooperation in the valley.
