Judy helps Holly Potter pick up her things from her Cousin Cleo's house, including a chest that Holly refuses to open because it is "forbidden." Cousin Cleo is not at home when Judy and Holly arrive, but since Holly does not care to wait, they leave without seeing Cousin Cleo. The next day Cousin Cleo accuses Holly of theft because all of her valuable antiques have been stolen. Judy knows that Holly did not take the antiques since Judy was with Holly at Cousin Cleo's home. Cousin Cleo is convinced that Holly has hidden her antiques inside the locked forbidden chest, but Holly refuses to open the chest to prove her innocence. Holly's mother's last words before she died were "Don't open it!" and so Holly will not open the chest. Unfortunately, Cousin Cleo continues to suspect Holly of the theft. Judy must prove Holly's innocence and the only way is to find the real thief, even though Judy has very few clues.