Captain Underpants and the Perilous Plot of Professor Poopypants

Captain Underpants and the Perilous Plot of Professor Poopypants is the fourth book in the "Captain Underpants" series written by Dav Pilkey.
Plot.
A scientist from the country of New Swissland (a country that doesn't exist, with a foreign culture where everyone has a silly name) called Professor Pippy P. Poopypants goes to the United States to demonstrate how his Shrinky-Pig 2000 and Goosy-Grow 4000 can help the world by reducing garbage and increasing food, but everyone laughs at Poopypants' silly name rather than take him seriously.
Meanwhile, Jerome Horwitz Elementary School is going to a restaurant-arcade called Piqua Pizza Palace, but when George and Harold rearrange the sign, Mr. Krupp catches the boys as he bans them from the school field trip and demands they clean the teachers' lounge instead. However, the boys get their revenge by modifying things around in the teachers' lounge and after the field trip, the teachers fall into George and Harold's trap and get largely covered in powder paste and Styrofoam pellets. The teachers chase the boys around the school causing Mr. Fyde the science teacher to see the teachers and think they're abominable snowmen. Mr. Fyde quits the next day, thinking he's losing his sanity due to this and other previous events in earlier books (including seeing Captain Underpants), prompting Mr. Krupp to find a new science teacher. 
Meanwhile, Professor Poopypants sees an ad to teach at the school and applies for the job, thinking children to be kind and sweet-hearted, but they spend days laughing at his silly name instead of learning anything. The professor only gets them interested by building a robot that makes gerbils jog along with them, but the interest is short-lived when George gets him to reveal his middle name (pee-pee). Some time later, Ms. Ribble reads The Pied Piper of Hamelin, which inspires George and Harold to make a comic about the Professor trying to take over the world using an army of gerbils in suits, which makes Professor Poopypants furious and destroys the last of his sanity.
He makes the gerbil machine as large as a tall building, climbs inside, then shrinks the school and holds them hostage. He then grows a pencil large and writes a system of three alphabetical name charts based on the first/last letter of each part of a first and last name to force people to change their names to silly ones (for example: Benny Krupp = Lumpy Pottybiscuts). George and Harold (now Fluffy Toiletnose and Cheeseball Wafflefanny respectively) get Captain Underpants (now Buttercup Chickenfanny, but refuses to take the professor's order to change names) to steal Professor Poopypants' Goosy-Grow 4000, but he and the machine are shrunk in the process. The two of them try to enlarge the school back to normal size, but get flicked off the school by Professor Poopypants. Cheeseball makes a paper airplane that Fluffy enlarges, allowing them to fly away, though they face many dangers, barely escaping a woodchipper, a dog, and a steamroller, until Captain Underpants rescues them. Fluffy then enlarges Captain Underpants (and Cheeseball's hand) to the robot's size, then they fight until Captain Underpants defeats Professor Poopypants and everyone's names are changed back to normal. The boys use the Shrinky-Pig 2000 and Goosy-Grow 4000 to bring themselves, the school (and everyone inside it) and Captain Underpants back their normal sizes. Captain Underpants then dresses and is soaked with water, turning him back into Mr. Krupp. 
Professor Poopypants is then arrested and hauled off to jail for his crimes. From the advice of George and Harold, he legally changes his name so that no one will make fun of it anymore. Unfortunately, he changes his name to that of his maternal grandfather's, Tippy Tinkletrousers, that which only makes the prisoners (and police officers) laugh at him even more, much to his anger. Later in the treehouse, George and Harold reflect on the moral to not make fun of people, believing this to be their first story to have a moral.
Characters.
Authors:
Following book.
The book that followed this was The Wicked Wedgie Woman (2001).
Reception.
The Book received mainly positive reviews from critics. 
The main antagonist of the book, Professor Poopypants, would return in later books of the series as both a recurring character and even the main villain. He would also return in .
The book was removed from an elementary school in Page, North Dakota due to parental complaints.