Harold and George have a passion for making comic books. They have invented a superhero, Captain Underpants. Their school principal, Mr. Krupp, can't stand the boyish pranks that Harold and George are capable of. But he has no proof that they were the initiators of many pranks. But now he does. The nerd Melvin filmed Harold and George reprogramming his invention, the Turbo Toilet, and turning a serious event into an entertainment for schoolchildren. Krupp wants to punish his friends. And then there is Professor Pooperpops, who wants to deprive mankind of a sense of humor. He is aided by Melvin and a giant-sized Turbo Toilet. Then Harold and George turn Director Krupp into Captain Underpants. Will the friends succeed in stopping Pooperpops? Will Krupp be able to run the school in his underpants?