The boys recap the important points of the plot with a comic of their own. Then, we again meet George, the kid with the tie and flat-top, and Harold, the kid with the t-shirt and the bad haircut (Remember that now.). We establish their rivalry with their teachers and Mr. Krupp, their general good nature, and their love of pranks.
George and Harold eagerly await this years Invention Convention, first prize being the opportunity to be Principal for a Day. Krupp contends that the "sticky chair incident," of last year, where the boys glued everyone to their chairs with a unique new glue, is enough to put them out of the running, and puts them in study hall for the whole day.
George and Harold set out to sabotage the inventions at the invention convention, but run into Melvin Sneedly, the school brain, while there, and make him promise secrecy in exchange for not sabotaging Melvin's invention, a photocopier that can make living, breathing copies of whatever you put in it. Melvin demonstrates with a photo of a mouse, but George and Harold disbelieve him.
The two boys sabotage the inventions, and the next day, Krupp hosts, his reassurances for naught: a dog washer sprays India ink, a ping pong ball server shoots eggs, a volcano detector shoots butterscotch pudding, et cetera. Melvin rats George and Harold out, and the two boys are given detention. After easily completing a chalkboard assignment with a jury-rigged writing tool, the two create a comic:
A UFO appears, animating the schools toilets! They eat the gym teacher and wreak havoc! Captain Underpants chokes them with plungers! The UFO spits out a turbo toilet and Captain Underpants gives it a wedgie, and sling shots it into the air! It blows up the spaceship, reverting the toilets to normalcy. Even the gym teacher survives.
George and Harold decide to make copies of the book to sell at recess. Teachers are in the office, so they decide to use Melvin's photocopier in the gym. When they do, it creates a series of evil, talking toilets, just like in their book!
Mr. Krupp spots the boys fleeing the gym and delightedly declares them suspended. All the teachers are overjoyed, and head to the gym to throw a party, unleashing the talking toilets, who eat Mr. Meaner, the gym teacher. Miss Ribble snaps her fingers, and Captain Underpants is unleashed... and apparently, he has a plan for the toilets.
