A plague of locusts strikes the Great Valley, decimating the plants and forcing the dinosaurs to migrate in search of food. When the adults in the herd begin fighting over the scarce resources, young Littlefoot and his friends Petrie, Cera, Ducky and Spike split off from the main group, becoming stranded on an island after their path was destroyed by a tsunami. As they search for a way off the island, they encounter an old acquaintance who may become friend or foe.

While Jane Holman is driving with her two sons, she accidentally runs into a drifter, Jack McCloud, who breaks his leg. Being responsible, Jane invites Jack, and his dog, to stay at her home until his leg has healed. Jack struggles to adapt their lifestyle, and finds himself loved by the family.