An earthquake panics Bronty, sending him from his cave. Dino Boy and Ugh follow their frightened pet into a mysterious vally where Bronty is captured by a 50-foot tall giant caveman, who puts him on a leash. The duo arrive to free their brontosaurus pal and do so, just as the caveman wakens and hurls huge boulders at the fleeing trio. As they make their way through a small opening, another earthquake fortuitously happens, sealing the giant back up in his valley forever.