Boober's been testing belouviouses, which are the part of a Fraggle's tail that flares up when scared. And when he tests Wembley's, Wembley wants to get even, so Philo and Gunge give him ideas on how to test his belouvious back. When hiding in the laundry and shouting "Boo!" doesn't work, he goes with an elaborate plan of setting up scary masks, spiders and snakes. Wembley goes back to Philo and Gunge to tell him of what he's set up, Marjorie tells him that when the boys tried scaring her, she nearly blew up. Wembley misunderstands this, and tries to go back to tear down his plan before Boober shows up, but he's too late, and thinks that Boober's blown up. Did Wembley go too far?
