In an alternate world where dinosaurs never became extinct, an Apatosaurus family is busy preparing their farm for the upcoming season. The youngest of the family, an anxious and undersized Apatosaurus named Arlo, struggles to make his mark on the family's legacy. His parents, understanding his fears, give him a responsibility: to guard their silo from pests. His attempt to fulfill this duty leads to a fateful encounter with a young human boy, who Arlo blames for a significant family loss. In a fit of fear and anger, Arlo chases the boy, but their confrontation ends with both of them swept away by a raging river.

Far from home and injured, Arlo must confront his fears as he embarks on a perilous journey through unfamiliar territories. Challenged by the enormous and untamed wilderness, Arlo forms an unlikely friendship with the young boy, who he names Spot. As they travel through a land teeming with prehistoric creatures and natural dangers, Arlo learns about the power of confronting his fears and the importance of perseverance. Amidst the journey, he discovers the true meaning of heroism and the significance of leaving his own mark on the world.