A young caveman (Robert Vaughn) wants to cross the river to where there's more game--but the ancient laws of the tribe prohibit this, saying that there's certain death. But he goes there anyways and sees not only lots of animals--but giant lizards. His father eventually admits that he made such a journey there when he was younger.
There's plenty of stock footage from other Roger Corman movies--and a weird man-in a costume looking monster that turns out to be an old guy in a radiation suit. In the final scene the cavemen find a book tucked into the suit--and it has pictures of our modern world. This isn't the past--it's the far future, after WWIII!