A mad scientist named Varko (guest star Mike Road) has used technology to convert 20th century Earth to a "Planet of the Neanderthals". How he did it is a mystery the Super Friends have to uncover.
As I'm sure you could tell, Casey Kasem (Robin) & Danny Dark (Superman) doubled as cavemen. The same likely for the rest of the regulars. Road had previously worked with Shannon Farnon (Wonder Woman) on Valley of the Dinosaurs three years earlier, but would shift to DePatie-Freleng for The New Fantastic Four a year after this episode aired.