Zosia was seven years old and curious. Her brother Olek was five. On Saturday they went to the small library. The cat Brush followed them like a shadow. Mrs Hela handed Zosia an old book. Inside lay a small golden key. It was no ordinary bookmark, oh no. The next morning the book disappeared from the shelf. Zosia found the key in her backpack. She couldn't remember how it had ended up there. There was also a card at the bottom of the backpack. On the card someone had written: "The door grows in the shade of a birch tree." Their backyard had one old birch tree. The children left with a cat and a key. The sun slowly moved the shadow along the wall. The shape of a small door appeared in the shadow. A brush suddenly meowed and a piece of brick moved. Zosia inserted the key into the cold hole. They heard a quiet click and something moved....