Clawtooth Mountain is a giant mountain range where Arlo and his family live. Near the base of the mountain is a river, which is the only way Arlo can determine where to find home if he ever gets lost. While trying to find a feral cave boy Arlo failed to kill out of reluctance, Poppa Henry and Arlo started to follow its tracks. When Arlo slips and injures himself, Poppa Henry decides to head back home. However, a giant mudslide arrives, and the two struggle to escape before they're washed away. Arlo is able to escape, but unfortunately, Henry doesn't, and he is washed away and killed.
When Arlo spots the cave boy again, he chases after it, blaming it for the death of his father. However, when it climbs up on Arlo, he panics, making him lose his balance, and fall into the river, where he is washed away from home.
After regaining consciousness, Arlo realizes he's far away from home. Remembering his father's advice, he follow's the path of the river to find his way back home. After a fierce rainstorm, Arlo loses the river, so he tries to look for the mountain. After being chased by a group of Pterodactyls, Arlo and Spot (the name of the cave boy) are saved by two Tyrannosaurus children named Nash and Ramsey, their father decides to help Arlo find the mountain so he can get back home. After following the three with their herd of buffaloes, Arlo finds the mountain, and bids them farewell so he can return home.
Unfortunately, as Arlo and Spot arrive to the mountain, the Pterodactyls attack and kidnap Spot, prompting Arlo to chase after them to save him. After defeating the Pterodactyls, a massive mudslide is approcaching fast, and Spot is in the oncoming path of destruction. Arlo jumps in, and successfully retrives Spot before he is washed away. The two fall down the giant waterfall, and begin heading home.
As soon as Arlo sees his farm while on the mountain, a family of cavemen arrive, and upon seeing Spot, Spot checks them out, where the family appears to adore him. Seeing how Spot is happy with the cave people, Alro decides to let Spot stay with them, despite the fact that Spot doesn't want to leave Arlo. Out of integrity, Arlo draws a circle around Spot and the family, making them a whole family. Seeing what Arlo is saying, Spot gives him a tearful goodbye before leaving with his new family, while Arlo goes back to his own family.