Brigid Lucy tries to be good, but it doesn't always work. This could be due to the invisible imp hiding in her hair. When Biddy's pet slug dies in tragic circumstances, Dad promises to buy Biddy a new pet. But Dad is allergic to almost every pet in the pet shop and the consequence is a menagerie of mayhem! Things are made even worse when the invisible imp in Biddy's hair decides to get involved. She can't help but encourage Biddy into trouble.