Captain Underpants and the Perilous Plot of Professor Poopypants is the fourth book in the Captain Underpants series written by Dav Pilkey.
It is the first book to have Captain Underpants to fully use his superpowers. In 2012, the ninth book, Captain Underpants and the Terrifying Re-Turn of Tippy Tinkletrousers had the main villain, Professor Poopypants (then known as Tippy Tinkletrousers), have his revenge and be the main villain yet again.
This book is dedicated to fellow children's book writer Cynthia Rylant (Buttercup Gizzardsniffer).
Professor Poopypants, a scientist from a country where everyone has a silly name, New Swissland, has just invented a shrinking machine, the Shrinky-Pig 2000 and an enlarging machine, the Goosy-Grow 4000. He goes to America so he can show these inventions to everyone and help the world by using his machines to shrink the world's garbage and increase the world's food supply.
Meanwhile, the kids in Jerome Horwitz Elementary School are going on a field trip to a pizza place (similar to Chuck E. Cheeses). While George and Harold are waiting to get on the bus, they decide to change the school sign, until Mr. Krupp catches them "red-handed" since he has been hiding in the bushes next to the sign and thinks their sign changing is rude and offensive. As the boys' punishment, they are banned from the field trip and have to spend the whole afternoon cleaning up the teachers' lounge. The teachers laugh at them. Harold becomes saddened they won't get any fun, however George claims they can still have fun and comes up with an idea involving a ladder, a bag of powdered paste and a box of styrofoam packing pellets.
The two boys start to create a prank as payback against the teachers. First, they tape the water nozzle of the sink, to make it on when the water runs, second they fill powder of paste on the wings of the ceiling fan. Finally, they fill the fridge with millions of foam pellets.
When the students finally come back and go home, the teachers except Mr. Fyde (who is on bus duty) spend their time taunting at George and Harold, saying that they missed a good field trip. Mr. Meaner says that he was going to bring back a pizza, but, he ate it on the bus. When the teachers finish taunting the boys, they go off to the teachers' lounge. When the teachers go into the lounge to relax, they fall into George and Harold's trap. The teachers end up looking like abominable snowmen (they are covered with foam pellets that are stuck to their bodies due to them being wet and covered in powdered paste) and they run, scream, and chase George and Harold down the hall with Mr. Fyde looking at the window.
The next day, their old science teacher, Morty Fyde quits because he thought he had cracked up. First, he says a toilet ate him. Then he says that later he started hearing dogs and cats in his classroom, then he imagined the school got flooded with sticky green goo, and finally, he saw the other teachers chase George and Harold down the hall as "abominable snowmen". Mr. Krupp says that can all be explained as George and Harold's doing, but Mr. Fyde also insists that he saw some fat bald man in his underwear fly out a window (obviously Captain Underpants) which makes Mr. Krupp realize he has gone crazy since he has no proof that is George and Harold's doing. As Mr. Fyde dances in joy, Mr. Krupp wonders where he is going to find a new science teacher on such short notice.
Meanwhile Professor Poopypants has been trying to demonstrate his new inventions to the world, but nobody takes him seriously due to laughing at his silly name. Eventually he begins running out of money to become a teacher at a school. He then goes to a New York coffee shop and after finding an ad, he decides to become an elementary school science teacher. He is convinced that the innocence and sweetness of children will let them pay attention to him, but they all laugh at his name instead after he declares what it is.
A week later, Professor Poopypants manages to get the children interested in science by building a machine called the Gerbil-Jogger 2000 that allows gerbils to jog alongside their masters. However, this interest is short-lived (thanks to George), as he reveals his middle name to be Pee-Pee, causing the children to start laughing again. Ms. Ribble reads the story of The Pied Piper from Hamelin. This inspires George and Harold to make a Captain Underpants comic book about the Professor inventing an army of gerbil joggers to destroy the world using a headphones on the gerbils to control them and everyone in the comic including Captain Underpants laughs at his name, which destroys the last of Professor Poopypants' credibility with the students and drives him crazy (after he found a copy of the comic in the hallway).
Using his enlarging machine, he makes the gerbil machine as large as a tall building and then gets inside. He then shrinks the school and holds them hostage. He then unveils his plan to force everyone in the world to change their names so everyone will have a silly name instead of a normal one, and anyone who refuses will be shrunk. Professor Poopypants does this first on Mr. Krupp. As a result, Mr. Krupp becomes Lumpy Pottybiscuits. All the children find this funny, until Professor Poopypants forces them to use the chart to change their names as well. This leaves the students and staff to be quite shocked and almost horrified at their new names.
George and Harold (now Fluffy and Cheeseball) had no choice but to get Captain Underpants's help knowing that they would regret it. So they rushed to Lumpy Pottybuscuits's (Mr. Krupp) office where he is seen under his desk hiding in fear. Fluffy snaps his fingers at him and he becomes Captain Underpants (now Buttercup Chickenfanny, but Captain Underpants refuses to change his name) to steal Professor Poopypants' enlarging machine, but unfortunately he gets caught and both him and the machine are shrunk in the process.
Which Captain Underpants's presences is unknown, Fluffy and Cheeseball try to enlarge the school back to normal size by getting onto the roof, but they get flicked off the school by the Professor. Cheeseball makes a paper airplane that Fluffy enlarges. The plane flies into many threats, yet tiny Captain Underpants rescues them and they end up in an abandoned alley.
Fluffy tries to enlarge Captain Underpants, but he accidentally enlarges Cheeseball's right hand in the process, however Captain Underpants does become visible, George zaps C.U. until he is the size of Professor Poopypants' gerbil machine. Captain Underpants then fights and defeats Professor Poopypants. Once Professor Poopypants is defeated, everyone gets their names back. The boys use the shrinking and enlarging machines to get themselves, the school and Captain Underpants back to normal size. Then Captain Underpants gets dressed back up like Mr. Krupp before being soaked with water, turning him back to his old nasty self again.
Professor Poopypants is then hauled off to jail. George then questions Professor Poopypants wouldn't it have been smarter to try changing his own name instead of forcing the rest of the world to change theirs. The surprised professor notes he never thought of that. As per George's advice, Professor Poopypants changes his name so no one will make fun of it anymore. Sadly, he chooses his grandfather's name (on his mother's side), Tippy Tinkletrousers, which the people at jail make fun of (including the guards).
The moral of the story ("I think this is the first time one of our stories had a moral," notes George) is that it's not nice to make fun of people.
George and Harold unfortunately forgot another moral in the story which was do not hypnotize the school principal because anything can go from bad to worse at the snap of a finger. Mr. Krupp then turns back into Captain Underpants (thanks to Ms. Anthrope) with George and Harold holding onto his cape saying "Oh No!" and "Here we go again!".