Arlo is an Apatosaurus, an enormous dinosaur that lives in a different world, because what the tape shows us is an Earth in which the meteorite that exterminated the dinosaurs did not fall.
That makes the dinosaurs do not become extinct and therefore, live with humans. In fact, Arlo befriends a young human, a strange friendship, but at the same time very endearing.
Both have to face a trip with uncertain result, because it is true that at that time the dangers lurk behind each tree. For example, Arlo and his human friend have to face other dinosaurs who want to eat them.
The trip will serve Arlo to learn a lot about himself. He has always been considered a coward, but along the way he is going to be surprised, discovering things about him that he had never imagined.