Having changed the course of human history after unwittingly wondering what could be worse than going to prison, George and Harold are shot through time back to kindergarten, where they are subjected to bullying and a clueless school principal.

When George and Harold hypnotize their principal into thinking that he is the superhero Captain Underpants, he leads them to the lair of the nefarious Dr. Diaper, where they must defeat his evil robot henchmen.

Harold and George's time-traveling Purple Potty lands them in an alternate universe where everything seems delightful, but evil lurks just beneath the surface, and leads to a battle between Captain Underpants and his own evil twin.