The Addams Family meets Nancy Drew in this delightfully spooky illustrated middle grade mystery following a young human girl determined to protect her monster foster family from discovery. The first rule of the Monstrous League of Monsters is to keep monsters hidden from humans. But when zombie George and his cat companion Bandit find an abandoned human baby, they can't leave her to be eaten by hobgoblins. So they spirit her home where she quickly becomes part of the family.

Fast-forward ten years, and young Theodora doesn't seem too scarred by her monstrous upbringing. But a series of anonymous letters suggests that someone is about to reveal the secret of her caretakers' identity. If Theodora doesn't act fact, she may lose her beloved monster family forever.