Littlefoot notices a Longneck herd entering the Great Valley and informs his grandparents, who tell him that the Longnecks are their cousins and that they must go to greet them. Upon the herd's arrival, the Old One, the herd's leader, tells the Great Valley's residents that her herd has been migratory ever since a heavy period of rain turned their old home into a marshland called "The Land of Mists" and became the home of many dangerous creatures. Later, Littlefoot meets a female Longneck 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.
Meanwhile, Grandpa becomes ill. The Old One notes that she is familiar with the illness, which is lethal to any dinosaur who does not eat the golden petals of the "night flower" soon enough. The night flower only grows in the Land of Mists, and the other Longnecks refuse to take Grandma there. Though Littlefoot has been warned that the Land of Mists is too dangerous, he becomes determined to save his grandfather and asks Ali to take him there. She agrees on the condition that Cera, Ducky, Petrie, and Spike do not come along, claiming that they will slow them down. As the two of them make their way, they pass through a cave and a sudden earthshake causes various stalagmites and stalactites to crash down, separating them. Ali returns to the Valley and convinces Littlefoot's friends to help her free him. Meanwhile, Littlefoot tries to find a way out and meets an old Archelon named Archie, who helps him dig through the rocks.
As they work, Littlefoot and Archie are interrupted by a mostly-blind Deinosuchus named Dil and an Ichthyornis named Ichy, who intend to eat them. Dil and Ichy pursue Littlefoot and Archie, but are knocked out when Ali and Littlefoot's friends dig a hole in the rock wall and send rocks tumbling down on them. Archie shows Littlefoot and his friends a way to the Land of Mists and reminds them to stay close together before departing. In the Land of Mists, Cera is separated from the group. The others meet a Megazostrodon that Ducky names Tickles because of his fur, which tickles her when she hugs him. Immediately after, Tickles helps them find Cera. However, Cera falls into a river and is pursued by Dil and Ichy, who have now recovered from their prior knockout in the cave. Ali saves Cera and distracts the predators, after which Cera softens towards Ali. Later, Ali explains her prejudicial behavior, having never interacted with species outside of her own, but remarks that her attitude towards Littlefoot's friends has now changed.
When the seven stop for the night, they realize they are in a field of night flowers after they bloom, and quickly stock up on them. As they make their way home, Ichy and Dil return for another attack. During a chase, Petrie is grabbed by Ichy but rescued thanks to Tickles, who tricks Ichy into biting Dil's tail rather than Petrie, giving him time to rejoin his friends. Dil becomes upset at Ichy, but they quickly resume the chase, during which Ducky falls into a river and becomes unconscious. Spike then speaks for the first time by calling out Ducky's name, which awakens her as she is about to be consumed by Dil. Ducky escapes, and Spike uses his tail to knock Ichy toward Dil's open jaws as she tries to consume Ducky.
Dil chomps Ichy by accident, mistaking him for Ducky, and the two get into another argument. The two declare that they have had enough of each other and should separate forever. Immediately after Dil leaves Ichy, she is pursued by a Hydrotherosaurus, who unknowingly saves Littlefoot and his friends by chasing Dil away. Dil desperately cries out for Ichy while being chased.
With the predators gone, Littlefoot and his friends say goodbye to Tickles and head home. They give Grandpa the flowers to eat, and he fully recovers a few hours later. Ali then leaves with her herd, but not before trying to convince Spike to say goodbye, which he does not, as he is too interested in consuming leaves. The narrator concludes that Littlefoot and the others would indeed meet Ali again one day.
Sound Effects Used
Sound Ideas, CARTOON, LAUGHTER - CHIPMUNK LAUGH, HUMAN (heard once in the Universal Cartoon Studios logo, 2003-present printings only)