Long story short, they discover that they've been swept away by a comet to a land inhabited by close-up shots of normal sized animals. All perfectly sound, scientifically speaking.
Well, our American and Frenchmen are separated and end up living with competing tribes, each earning the affection of a hot young cave girl. When Frenchman's hot blond cave girl is captured by the American's tribe, our hero realizes his former adversary is still alive and goes with her to reunite them. When a volcano erupts trapping some of the American's tribe in a cave blocked by close-up shots of normal sized animals, the grateful Frenchman rallies his tribe to the rescue. He invents gunpowder to kill the Gorn. Hope he doesn't get tinnitus. Body count: 2.
So, what have we learned here? We've learned that people from different nations can live in peace so long as they have equal numbers of hot cave girls. Also, modern humans can gain status in primitive tribes using their superior intelligence, even though they lack all the skills required to survive in a primitive world.
What a plucky little film! Fun, imaginative, well-paced, short, brain-dead, all the things I look for in B horror and sci-fi movies. Don't confuse it for an important or influential piece of cinema, but do take a look. If for nothing else than the gratuitous swimming scene. AMRU 3.5.