In the third volume of this critically acclaimed series, Mousse learns during a visit from his niece that some treasures are truly priceless. Mousse has meticulously prepared for his niece Pistachio's arrival. He's gone to the bakery to buy fresh bread and cheese. In her room, he's set out a fresh notebook, with pens and colored pencils, and put together a small library with his favorite books. But Pistachio has plans of her own: she only wants to play in the waves. When Mousse receives a letter from his friend Barnacle about a rummage sale, he and Pistachio rush to the attic to prepare, stumbling upon the treasures hidden in his old house. What memories do they hold?