George and Harold are up to their old tricks again, and when their latest prank makes a direct hit on school brainiac Melvin Sneedly, he decides to get even! But in the blink of an eye, and the sneeze of a nose, Melvin goes from snooty to snotty, and the Bionic Booger Boy is born! Can Captain Underpants clean up this catastrophe, or will the mucous monster make a mess of things?