Captain Underpants and the Perilous Plot of Professor Poopypants is the fourth book in the Captain Underpants series written by Dav Pilkey. The book is about a mad scientist named Professor Pippy P. Poopypants becoming a new science teacher at Jerome Horwitz Elementary. However, all the students laugh at his name.

A scientist from the fictional country of New Swissland named Professor Pippy P. Poopypants goes to the United States to demonstrate how his Shrinky-Pig and Goosy-Grow inventions can help the world, but everyone laughs at Poopypants' name instead of taking him seriously.

Meanwhile, Jerome Horwitz Elementary School is going to a restaurant-arcade called The Piqua Pizza Palace, but when George and Harold rearrange the lunch sign, Mr. Krupp demands they clean the teachers' lounge, making them lose their chance of going to the Piqua Pizza Palace with everyone else. However, the boys get their revenge by modifying things around in the teachers' lounge and after the field trip, the teachers get largely covered in glue and foam pellets before Ms. Ribble makes it even worse. They chase the boys around the school and after seeing the teachers looking like abominable snowmen, Mr. Fyde retires, thinking he's going insane. Meanwhile, Professor Poopypants sees an ad to teach and applies for the job, thinking children to be kind and sweet-hearted, but only gets them interested by building a robot that makes gerbils jog along with them. 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, which destroys the last of Professor Poopypants' sanity.

He makes the gerbil machine as large as a tall building, then shrinks the school and holds them hostage to turn their names sillier, with a system of 3 alphabetical name charts based on the first/last letter of each part of a first and last name. George and Harold's names are changed to "Fluffy" and "Cheeseball" respectively. They get Captain Underpants to steal Professor Poopypants' enlarging machine, 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. Harold makes a paper airplane that George enlarges and after many dangers, Underpants rescues them. George enlarges Underpants to the gerbil's size, then he defeats Professor Poopypants and everyone's names revert. The boys use the machines to bring the school and Captain Underpants back to normal size. Captain Underpants is soaked with water, turning him back into Mr. Krupp. Professor Poopypants is hauled off and from George and Harold, he changes his name to his grandfather's, Tippy Tinkletrousers, that which makes the prisoners ridicule him even more, much to his anger.

The book following this was "Captain Underpants and the Wrath of the Wicked Wedgie Woman," released in 2001.

The book was removed from an elementary school in Page, North Dakota due to parental complaints.^[1]

• Dav Pilkey
• Captain Underpants
• Captain Underpants: The First Epic Movie, the Captain Underpants movie, with a plot similar to the book