In a fascinating alternate history, an asteroid that would have led to the extinction of the dinosaurs 65 million years ago safely passes by Earth, allowing these magnificent creatures to thrive.
Fast forward millions of years, we meet Apatosaurus farmers Henry and Ida, who are raising their three children: Libby, Buck, and Arlo. While his confident siblings find success and are allowed to leave their mark—a symbolic mud-print on the family’s corn silo—Arlo struggles with his timid nature, making simple tasks feel daunting. In an effort to instill a sense of responsibility in his son, Henry assigns Arlo the role of guarding the silo and helps him set a trap. When they capture a feral caveboy, Arlo, unable to kill the creature, chooses to set him free, leading to Henry's disappointment and anger.
To teach Arlo a lesson, Henry takes him on a journey into a ravine to track the escaped caveboy. Tragically, during this expedition, Arlo's father sacrifices himself to save him from a devastating flash flood.
With Henry gone, Arlo is now burdened with increased responsibilities. In his anger over the caveboy's role in his father's death, Arlo chases him into a river, and both get swept away, leading to a journey that changes Arlo forever. After losing consciousness from a blow to the head, Arlo awakens deep in the wilderness, where he finds himself pinned under a boulder. The next day, he discovers the caveboy, named Spot, digging to free him and offering food. Spot’s bravery is evident when he defends Arlo from a large snake, which captures the attention of Forrest Woodbush, a quirky Styracosaurus who decides to keep the caveboy for himself. Arlo impressively wins a competition to name the caveboy "Spot," and as they bond over their homelands, Arlo shares his grief over his lost family, revealing that Spot has also lost his parents.
Their adventure takes a turn when a storm frightens Arlo, and he becomes separated from the riverbank leading him home. With Spot at his side the following morning, they are soon detected by a dangerous group of carnivorous pterodactyls. As they attempt to eat Spot, Arlo and Spot flee, only to encounter Nash and Ramsey, two Tyrannosaurs. After teaming up with Butch, who has also faced loss—the disappearance of his longhorn herd—they confront a band of Velociraptor cattle rustlers. In a clever plan, Arlo and Spot lure the rustlers into the open, enabling Butch and his family to take back what belongs to them.
As night falls and they share stories around a campfire, Arlo participates in driving the cattle south and begins to see the towering mountains of his homeland on the horizon. Driven by a desire for home, Arlo sets off with Spot, encountering a feral caveman family along the way. While Spot is drawn to them, Arlo gently encourages him not to join, prioritizing their mission home.
However, impending storms bring back danger, and Spot is captured by the pterodactyls once more. Arlo is pushed into a treacherous situation where he is knocked out and has a vision of his father, Henry, guiding him back home. Realizing that his anger towards Spot is misplaced, Arlo vows to save him. With newfound determination, he battles the pterodactyls, ultimately rescuing Spot from danger.
They both tumble down a waterfall but Arlo protects Spot during the perilous drop. When they finally return to Arlo's home, the caveman family is eager to claim Spot as one of their own. Despite his reluctance, Arlo understands Spot's need for family and with heavy hearts, they bid an emotional farewell. Upon his return, Arlo finally leaves his mark on the silo, standing proudly between those of his beloved mother and father.
