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?