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.

George Beard and Harold Hutchins present their favorite stories about the superhero Captain Underpants, who comes to life when they snap their fingers at their school principal, Mr. Krupp, and whose adventures occur as the boys draw them.

George and Harold, and their doubles, Yesterday George and Yesterday Harold, have a good thing going. Two of them go to school, while the other two hide in the tree house and play video games all day-then they switch! But when their malicious gym teacher, Mr Meaner, creates a method of mind control that turns their fellow students into attentive, obedient, perfect children, the future of all humanity will be in their hands!

Two overly imaginative pranksters named George and Harold, hypnotize their principal into thinking he's a ridiculously enthusiastic, incredibly dimwitted superhero named Captain Underpants.