The break bell echoed through the corridors of Primary School No. 17. Pupils ran out of their classrooms like a storm, and laughter and conversation mixed with hurried footsteps. Lena, Tom and Victor met under the notice board as usual. The maths competition was approaching and each of them, although they didn't admit it out loud, hoped to win this time. But they had a different plan for today. The library was a place most students avoided, but for them it was an arena of adventure. They loved to rummage between the bookcases, finding the strangest books and jotting down tidbits of information, which they then regaled each other with at breaks. On this day, Mrs Barbara, the librarian, only nodded as they sneaked quietly through the door. - Are we looking for something for a project? - She chuckled with a smile, without asking for details. - 'Something like that,' Tom replied, glancing at the non-fiction shelf. Victor had already disappeared among the shelves, and Lena followed him, attracted by the sight of the new 'non-obvious books' section. They first noticed a small green book squeezed between two bulky volumes on the history of the city. It was untitled and the spine was plain. Lena carefully pulled it out. - 'Hey, check this out,' she whispered, and the boys joined her. Upon opening it, they found that each page contained only a few numbers and letters, written in a special cursive script: 12F-7, 21A-3, 15D-11. - Maybe it's some kind of code? - wondered Viktor, scratching his head. They started leafing through books from the same section. It turned out that they had found similar markings in the margins of several other volumes, written in barely visible pencil. They all looked as if they led to something in the library - specific shelves, perhaps? For the next few minutes they compared the numbers and letters with the book signatures on the shelves, until Lena called out in a whisper: - Look! 12F-7 is the number of this shelf and place seven.... Oh, here's something! She slipped her hand between the books and pulled out a small envelope. Lena's heart was beating like crazy and the boys looked at her with wide-open eyes. Inside was a piece of paper on which someone had written one short sentence: "If you are reading this, you already know where to look - but time is working against you. The next step awaits you in room 204." Tom looked uncertainly at Lena and Victor. - Room 204? Nobody ever goes there, it's that old classroom at the end of the corridor. What if someone spots us? Victor just smiled broadly and Lena felt goosebumps. They decided to act. Slowly, almost without hearing their own footsteps, they left the library and headed towards room 204. As soon as they swung the door open, something bright shone inside. Suddenly, the silence was broken by a strange sound, and all three looked into the depths of the classroom, where a figure was leaning out from behind an old cupboard....