On Shadow Street, just off the park, stands an unusual school - a centuries-old, sprawling building covered in ivy and mystery. When the last bell sounded on Friday afternoon, Lena, Michal and Basia stayed at the school as the only students. Officially to help the librarian with tidying up the book collection, and unofficially to solve the mystery of the strange noises they had heard for several weeks on the stairs leading to the basement. The basement was a forbidden place - always locked, with a fairly new padlock shiny on the door. As soon as the librarian came out to lock the door from the language room, Lena convinced her friends to set off to investigate. - 'I heard something rattling under the floor in classroom 3B yesterday,' Lena whispered, leading the group down the empty corridor. - It wasn't just a rat! They reached classroom 3B. Everything seemed ordinary: the old benches, the chalkboard, the alphabet posters. But as Basia moved a bench against the window, something sounded metallically under Michael's foot. - There's something here! - he called out in a hushed voice. They pushed back the rug and discovered a shiny brass ring protruding from the floor. Michael pulled on it and a section of floorboards lifted, revealing a narrow, winding tunnel downwards. A celadon light shone from the darkness and a soft whisper filled the air, as if someone was waiting on the other side. The friends looked at each other uncertainly, but curiosity won out. Lena was the first to slip into the tunnel, and her legs disappeared into the unknown. Michael and Basia followed her, with their souls on their shoulders. There was a strange chill inside, and the walls shimmered, reflecting unknown signs. After a few steps, the tunnel opened into a spacious underground hall full of old desks and wooden boards covered in mysterious writing. Above one of the benches hung a mirror in which were reflected faces completely unlike any of them. Suddenly, a quiet whisper came from behind the blackboard: - Hello, late students. We have waited a long time...