George and Harold are two imaginative pranksters who make the depressing Jerome Horwitz School a better place. They hypnotize their nemesis, the mean-spirited principal Mr. Krupp. He transforms into the incredibly clueless yet kind-hearted and enthusiastic superhero named Captain Underpants. Even without the mean principal, they discover that having Captain Underpants comes at a steep price. Meanwhile, Professor Poopypants tries to eliminate laughter with the school snitch Melvin, and George and Harold discover that there may be more to Mr. Krupp than they had originally thought..