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.

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.

When she is named Principal for the Day, Frannie wonders if principals assign their own punishment when they get in trouble for using office supplies or accidentally set off the fire alarm.

George, Harold, Captain Underpants, and Sulu the Bionic Hamster battle the three Ridiculous Robo-Boogers in an attempt to turn the Bionic Booger Boy back into Melvin Sneedly.

After the principal of P.S. 88 is turned into a frog by a bumbling magician, the frog bargains for the chance to act as principal until Mr. Bundy returns from his "family emergency."

"Mr. Keene loved being the principal of his fine, fine school. He was so proud of his students drawing and painting and reading and singing that he thought his buttons would burst! Then one day his appendix burst, and a new principal, Mr. Tatters, swooped in to take his place. Mr. Tatters said, 'In order to be a smart, smart school, we will have lots and lots of tests.' And suddenly there were no more cards and banners, posters and songs. Instead,...

The monstrous Turbo Toilet 2000 is determined to take revenge on George, Harold, and Captain Underpants.

When Mr. Lincoln, "the coolest principal in the whole world," discovers that Eugene, the school bully, knows a lot about birds, he uses this interest to help Eugene overcome his intolerance.

The Notebook of Doom is missing and Alexander and his fellow-monster hunters, Rip and Nikki, are convinced that the boss-monster has it; but is that monster the mysterious Principal Vanderpants, who is certainly behaving oddly, or someone else--and they need to figure that out quickly because a mistake could have deadly consequences for Stermont Elementary.

A boy hears all sorts of crazy rumors about the faculty at his school.