Littlefoot has nightmares which involve the sun, or "Great Circle". He mentions it to his grandparents, who share the experience. Grandpa has a feeling that he wants to "go somewhere", and they leave the next day with no help but sense to tell them where they were going. Out of curiosity, the others follow him, starting after them the following night. But there's disaster after disaster as waking them up isn't easy for Cera.
The movie does not explain a lot of things. For starters, a myth stating that longnecks saved the world. "The night circle (moon) was jealous of the great circle (sun) and pushed it out of the sky (a solar eclipse). A herd of longnecks were on the ground and 'they stretched their necks up and caught the great circle on their heads, and then pushed it right back up into the sky.'" Second, the crater is not explained.
New characters are introduced through the movie: "Sue", a supersaurus, who is driven on by the same sense as Grandpa, joins Littlefoot; and Cera and the others met "Pat" who is an eldery Apatosaurus. Sue is always haunted by a want to meet someone taller than her, as she towers above everyone else. Pat is slow-moving, but does arrive at the crater in time for the event. He injures his foot due to volcano activity.
However, another surprise is in store, as Littlefoot meets his father, Bron, for the first time in his whole life. However, Bron isn't alone. Since parting from his now deceased wife (a song from Weekenders Adventures of The Land Before Time IV: Journey Through the Mists reveals that the concept of marriage does exist in Weekenders Adventures of The Land Before Time universe), he has become guardian to another young Brachiosaurus, Shorty, who becomes jealous of Littlefoot since Bron is spending more time with him.
Bron's story is a long one. He left to find a new place to live before Littlefoot hatched, but only arrived to find that the Earthquake had destroyed everything. In search of Littlefoot's mother, he finds Shorty, but realises that it's not Littlefoot. But they stay with him. And then he is joined by others. This repeating process makes him the leader of a herd. And the same sense brings them to the crater.
Meanwhile Sarcosuchus and three T-Rexes the time comes for the Longnecks to save the sun again. Then they succeed and save the world. (However, the audience watching the film will recognize that what happened was, in fact, a solar eclipse; the Longnecks merely thought the sun was falling.)
With their mission completed, the herds depart. Knowing their true places, Littlefoot tells Bron to take care of Shorty, and returns to the Valley with his friends and grandparents.