One Million B.C.

A group of modern-day hikers is caught in a raging storm and takes shelter inside a dark cave. An accompanying anthropologist studies ancient carvings carved into the rock and uses them to frame the legend of a young caveman. The carvings hint that the events to come are the unfolding tale of prehistory rather than mere myth.

In the Rock Tribe's hunting grounds, Akhoba commands the hunt and the tribe's power. Tumak fights for the right to his first kill, a small Triceratops, and the tension within the group reflects the strict social order. An elderly hunter is left to die after a dangerous fall, underscoring the ruthlessness of the tribe's customs.

The tribe returns to their cave with the quarry, and the ritual of sharing the kill exposes the power struggle. Tumak defends his portion, and Akhoba shoves him from a cliff in a brutal confrontation. A mastodon attack soon follows, sending Tumak downstream toward a different world with the Shell Tribe.

Tumak floats unconscious along a river after the fall and is discovered by Loana of the Shell Tribe. He is taken to their cave where the tribe shares a meal of vegetables and other simple foods. Tumak notices their unlike customs—sharing, order, and cooperation—rather than the harsh competition he knows.

While Akhoba is injured in a hunting accident, a younger hunter asserts authority and steps into leadership, leaving Akhoba crippled and sidelined. Akhoba later returns to the cave to find himself treated with contempt by the others. The balance of power within the Rock Tribe shifts in the wake of his decline.

Tumak slowly adjusts to life with the Shell Tribe, helping the children gather fruit and learning their calmer ways. They teach him to laugh and to share, opening him to a different code of social exchange. The emotional transition marks a clear contrast to his former life among the Rock Tribe.

While attempting to fish, Tumak struggles with Shell hunting methods. An Allosaurus traps a child in a tree, and Tumak borrows a spear to kill the dinosaur and rescue the child. He refuses to return the spear to its owner, hinting at his stubborn, proud nature even as he becomes a hero.

Following his bold rescue, Tumak steals the spear and a hammer from the tribe’s maker and assaults him when questioned. The Shell Leader banishes him, ending his life among the Shell Tribe for now. Loana, who has grown fond of him, is distraught yet remains with him as he departs.

Despite the banishment, Loana travels after Tumak, drawn by love and a shared destiny. They push onward together, crossing the boundaries between the Shell and Rock worlds. Their bond strengthens as they face an uncertain future.

Tumak and Loana arrive near the Rock Tribe but are caught in a fissure caused by a fight between roaring dinosaurs. Loana escapes the trap but is confronted by the Rock Leader who displaced Akhoba. She signals for help with her shell horn, drawing Tumak into action.

With Loana’s aid, Tumak defeats the tribe’s rival leader and earns the right to lead both groups. He reorganizes their meals to emphasize shared care—Loana serving women and children first—signaling a new era of cooperation. The union of the Shell and Rock Tribes elevates Tumak’s authority and the couple’s bond.

A nearby volcano erupts, destroying the caves and scattering both tribes. Loana rushes to save a child from the lava flow and escapes toward the Shell Tribe with the rescued youngster. Tumak searches for Loana and finds only a scrap of her clothing, believing she is lost.

Back at the Shell Tribe, a large iguana-like dinosaur traps the cave. Tumak answers a call from Loana alive and fights to defend her. Akhoba’s wisdom returns as he advises a distraction while others climb to higher ground, triggering a rockslide that kills the beast. The formerly despised Akhoba earns respect as the tribes begin to unite.

With the beast slain and Akhoba honored, the Shell and Rock Tribes join as one community. Tumak and Loana stand with the rescued child as symbols of a new era. The film closes on the dawn of a new day for their unified people.