When George and Harold hypnotize their principal into thinking that he is the superhero Captain Underpants, he leads them to the lair of the nefarious Dr. Diaper, where they must defeat his evil robot henchmen.

When Professor Pippy P. Poopypants comes to Jerome Horwitz Elementary School to teach science, and he goes off the deep end because students make fun of his name, Only Captain Underpants can save the school from the professor's perilous plot.

Only Captain Underpants can stop the three evil space aliens who have invaded Jerome Horwitz Elementary School and turned everyone into lunchroom zombie nerds.

George and Harold have played a trick or two on nearly everyone at Jerome Horwitz Elementary. When their latest prank causes the school's cranky cafeteria ladies to quit, Mr. Krupp hires a trio of unusual replacements, who happen to look an awful lot like aliens! Will that curtain-caped crusader, Captain Underpants, save the day once more? Or will those outer space cafeteria ladies have him for lunch?

A snotty situation ensues when Melvin Sneedley, an academically-gifted student, turns into the Bionic Booger Boy and must be stopped by George, Harold, and Captain Underpants.