George and Harold are in a sticky situation. Their latest prank has snotty school brainiac Melvin Sneedly seeing red, and he can't wait to get them back. But when Melvin tries to transform himself into a bionic-powered superboy, things get a little gloppy - and the Bionic Booger Boy is born! Can Captain Underpants beat this blobby behemoth of blech, or is the entire world doomed to drown in disgusting nose-dribble? Pass the tissues, please!