Littlefoot's grandfather tells the children about "The Lone Dinosaur", a legendary longneck who once protected the Great Valley, who is the basis for a huge monolith that the dinosaurs call "Saurus Rock." A few days later, Littlefoot accidentally falls off a cliff, only to be rescued by a strange longneck dinosaur named "Doc." Doc is the spitting image of Saurus Rock, and displays knowledge of the great valley... could the legend be true?