Principal Krupp once again turns into the superhero Captain Underpants in order to save the world, and Jerome Horwitz Elementary School, from the evil talking toilets and the Turbo Toilet 2000.

Allie West finds it challenging to make new friends and stay true to old ones as she navigates fifth grade at a school run by her mother.

Tired of following the strict rules of their grumpy replacement principal, second-grader A.J. and his classmates try to find a way to bring back eccentric but lovable Principal Klutz, who was fired for bringing a wild turkey to the school's Thanksgiving Day assembly.

While Raven is away, Luca breaks his promise to her and returns to the Wish Library, asking to be in charge at school, but being principal for a day is not all fun and games.

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

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.

"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 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.

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

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.