This movie is sometime before Littlefoot turns 20. It is when his long-lost uncle Kaha, his mother's brother, turns up in the Great Valley.
Kaha was supposed to have stayed with Littlefoot's mother and his parents, but he left them, asking them not to mention him again, after a big dispute about being friends with non-Longnecks. Bron was furious when he found out and Grandma and Grandpa Longneck kept their son's wishes.
Eventually, when Littlefoot is near 20, Kaha turns up in the Great Valley and Littlefoot finally learns that he has an uncle. However, Littlefoot and Kaha don't get along.
However, after being out in the Mysterious Beyond with Kaha, as well as his friends, Littlefoot learns that a plague has struck the Great Valley that is infecting Longnecks and will prove fatal if a special plant (not the Nightflower this time) is found. Thus, Littlefoot is forced to go with Kaha to save the Longnecks of the Great Valley.