Like the original film, this remake is largely ahistorical. It portrays dinosaurs and humans living together, whereas, according to the geologic time scale, the last dinosaurs became extinct roughly 65 million years BC, and Homo sapiens (modern humans) did not exist until about 200,000 years BC. Ray Harryhausen, who animated all of the dinosaur attacks using his famous stop motion technique, has stated that he did not make One Million Years B.C. for "professors" who in his opinion "probably don't go to see these kinds of movies anyway" (this was a comment he made for the DVD of the 1933 version of King Kong).
PlotTumak (John Richardson), a caveman from the Rock tribe, is banished to the harsh desert because of a fight with his father Akoba (Robert Brown the tribal leader). After surviving many dangers such as a giant lizard, ape men, brontosaurus and a giant spider, he collapses on a remote beach, where he is spotted by "Loana the Fair One" (Raquel Welch) and her fellow fisherwomen of the Shell tribe. They are about to help him when an Archelon (which is three times the size of the actual prehistoric Archelon) makes its way to the beach. Men of the Shell tribe arrive and apparently fend off the giant turtle with spears (Although in fact, it appears the turtle just wanders out to sea, where it originally intended to go anyway).
Tumak is taken to their village, where Loana tends to him. Scenes follow emphasising that the Shell tribe is more advanced and more civilized than the Rock tribe. They have cave paintings, music, delicate jewellery made from shells, and rudimentary language - all things Tumak seems to have never before encountered.
When the tribe women are fishing, an Allosaurus attacks. The tribe flees to their cave, but in the panic, a small girl is left trapped up a tree. Tumak seizes a spear from Ahot (Jean Wladon), a man of the Shell tribe, and rushes forward to defend her. Emboldened by this example, Loana runs out to snatch the child to safety, and Ahot and other men come to Tumak's aid, one of the men being killed before Tumak is finally able to kill the creature.
In the aftermath, a funeral is held for the dead men - a custom which Tumak disdains. Leaving the funeral early, he re-enters the cave, and attempts to steal the spear with which he had killed the Allosaurus. Ahot, who had taken back the spear, enters and is angered by the attempted theft, and a fight ensues.
The resulting commotion attracts the rest of the tribe, who unite to cast Tumak out. Loana leaves with him, and Ahot, in a gesture of friendship, gives him the spear over which they had fought.
Meanwhile, Tumak's brother Sakana (Percy Herbert) tries to kill their father to take power. Akoba survives, but is a broken man. Sakana is the new leader. While this is happening, Tumak and Loana encounter a battle between a Ceratosaurus and a Triceratops. The battle is eventually won by the Triceratops which fatally gores its opponent.
The outcasts wander back into the Rock tribe's territory and Loana meets the tribe, but again there are altercations. The most dramatic one is a fight between Tumak's current love interest Loana and his former lover "Nupondi the Wild One" (Martine Beswick). Loana wins the fight but refuses to strike the killing blow, despite the encouragement of the other members of the tribe.
While the cave people are swimming - seemingly for the first time, and inspired by Loana's example - they are attacked by a female Pteranodon. In the confusion, Loana is snatched into the air by the creature, and dropped bleeding into the sea, when a thieving Rhamphorhynchus intervenes.
Tumak initially believes her dead. Sakana then leads a group of fellow hunters in armed revolt against Akoba. Tumak, Ahot and other members of the Shell tribe arrive in time to join the fight against Sakana. In the midst of a savage hand-to-hand battle, a volcano suddenly erupts: the entire area is stricken by earthquakes and landslides that overwhelm both tribes.
As the film ends, Tumak, Nupondi, and the surviving members of both tribes emerge from cover to find themselves in a ruined, near-lunar landscape. They all set off - now united - to find a new home.
 Main cast