Fonz and the gang end up in dinosaur times. Richie and Ralph get kidnapped by...
The Fonz and the Happy Days Gang
Season 1 Episode 12: Science Friction
It's 3057, exactly 1100 years into the Happy Day's gang future. Civilization is split up into humans living above ground and Krolacks living underground. Unfortunately Richie and Ralph end up on the Krolacks' workforce.