At first we're shown a desolate desert landscape, then an oryx and a hyena. Finally, the cavemen, dressed in animal skins, appear. They kill the oryx but one of them is killed by the horns of its mate. The others quickly bury the guy. When they get back to the cave, where all the women and children are, the nearby volcano explodes. The ground begins to shake, the volcano erupts and it's mixed in with footage from Hammer's earlier cave-movie, One Million Years B.C., which is sort of goofy because the cavemen look completely different than the ones here.
After the earth-shattering event is done, the survivors fight as to who will be the new leader. The tribe splits and one half goes to new hunting grounds. They meet a tribe of blonde people and the leader is given a young bride and they move on. She gives birth to twins but dies. The two sons, one dark-haired, the other light haired, don't get along. They are shown fighting as children. When they grow up their father dies and they fight for rulership--and the blonde wins. But rather than kill his dark-haired brother he just wounds him.
This is an interesting flick in that there's no talking (except the made-up caveman language) in the entire movie. It's sort of a thematic sequel to Hammer's One Million Years B.C and When the Dinosaurs Ruled the Earth, adapting the whole "Cain & Abel" bible story into the screenplay. There are no dinosaurs but there is a scene with this weird tribe who wear clay masks on their heads. Recommended.