Fourth-graders George Beard and Harold Hutchins skate over ketchup packets in the school gym until their mean-spirited principal, Mr. Krupp, catches them and makes them write a 100-page essay on good citizenship.

The boys instead create a comic book about a superhero called "Super Diaper Baby", to Mr. Krupp's surprise.

He lands in Deputy Dangerous' container of juice and drinks it, gaining superpowers and becoming Super Diaper Baby.

The next morning, Deputy Dangerous invents a robotic ant mecha-suit and intends to use it to destroy the city as an act of revenge.

Billy and Diaper Dog order superpower juice at a restaurant on Uranus and give it to Captain Underpants, restoring his powers.

An advertisement for a sequel titled Super Diaper Baby 2: The Invasion of the Potty Snatchers is found on the last page of the first book.

[2][3] On the back page, there was a picture of the sequel with the words The Third Epic Novel by George Beard and Harold Hutchins underneath.

When the boys struggle to think of something to write that does not involve poop, Krupp shows them a book from his childhood, How the Grinch Stole Christmas!

Upon reading it, George and Harold are inspired create something without poop and decide to make their sequel about pee.

That night, Mr. Hoskins feels depressed, but after a reminder by his wife, he decides to read Mechafrog and Robotoad are Enemies to Billy.

The following night, Rip Van Tinkle puts Petey in the Robo Kitty 3000, before he evaporates and rains pee on people.

The pee drops unite to recreate Rip Van Tinkle, while Petey steals diapers to sell to the people who were robbed of toilets.

A drop of pee gets flicked up and lands in the Mayor's pool, where it grows into an enlarged Rip Van Tinkle, whom the duo finds wreaking havoc at the train tracks.

Barbara Schultz of Common Sense Media gave the first book three stars out of five, calling it "really pretty hilarious."

However, she also noticed the humor involved with "almost all the bathroom variety, and it is so funny to kids that they will read it and talk about it many more times than adults might want.

"[6] Super Diaper Baby was challenged in the Riverside Unified School District for being inappropriate for children due to its excessive toilet humor and intentionally misspelled words.