The Town Hall Library smelled of dust, ink and old wood. Lena, aged eleven, was sorting out old newspaper folders. Mrs Irena closed the study and went downstairs to get some tea. On the desk lay an atlas, open to a page about the city. Between the pages someone had slipped a folded, slightly greasy sheet of paper; Lena looked at the note carefully. It was a map of the library, with an arrow and a short inscription. The arrow led to Bookcase 17 and the caption: "Return at dusk". Next to it lay a small brass pin in the shape of a fox. It was getting late, but Lena didn't go home. She called Tymek, a neighbour from their townhouse, for help. - An adventure in the library? - He asked cheerfully and added: - I'll be there in five minutes. He arrived by bicycle, left his helmet on the chair and took off his gloves. The light dimmed as a cloud covered the tall windows of the reading room. Only the fast clock above the entrance ticked, strangely restless. Bookcase 17 stood in the reading room, between the city history and the scrapbooks. Lena counted the shelves in a whisper, while Tymek shone a torch from his phone. On the shelf with atlases, one book had a different spine. Lena pressed the protruding spine with her finger until the mechanism clicked. A paper rustled behind the back, as if someone had breathed in the folders. - Did you hear that? - she hissed, and the echo vibrated in the semi-darkness. - 'Maybe the wind,' he whispered. Behind the bookcase was a narrow corridor and steps down. A metal door with an engraved fox awaited at the bottom. The fox on the door was like Lena's pin. - If it's a trap, we'll turn back,' she said very calmly. She stuck the pin in the round hole, like a key. Something clicked and quietly buzzed in the lock. At the same moment, footsteps came from upstairs. The light in the reading room went out suddenly. Someone stopped at bookcase number seventeen and gently pressed the spine of a book. The door in front of them vibrated, and cool air came from inside. Then a whisper sounded, very distinct, just beyond the threshold: "Lena?"