The multi-level building of the Maria Skłodowska-Curie High School stood on a street full of chestnut trees for more than a century. Stories of secret passages, hidden staircases and forgotten rooms circulated among the students. However, none of these stories were as fascinating as what happened to Amelia, Tom and Erik. The last bell for maths class sounded while the three friends were still sitting on the windowsill in the empty corridor of the old wing of the school. Instead of running to class, they decided to have a quick look in the library for notes that someone had apparently left between volumes on industrial development. As they passed the bookcases, Erik noticed something strange - a soft light coming from under a door neither of them had seen here before. They were numbered 23, although there was no such class on the official school map. The door looked heavy, with a brass handle whose gilding had long since worn off. - Have you ever seen this? - whispered Erik, pointing at the strange door. - 'Never,' replied Amelia, feeling a thrill of excitement. Tom cautiously approached and touched the handle. The door did not budge. He tried harder, but nothing happened. Then Amelia noticed something else - just below the handle, at knee height, someone had scratched out tiny letters: "For the brave only". They did not think long. In unison, all three of them began to scour the nearby shelves and floor, looking for anything that could help open the secret entrance. After a while, Erik spotted a small silver key stuck between the pages of an old French-Polish dictionary. With a trembling hand, they turned the key in the lock. The door creaked and opened with little resistance. It was dark inside, but a quiet sound came from deep inside, as if someone was whispering.... They looked at each other. They knew they could not back out now. As they took the first step across the threshold of classroom number 23, indistinct shadows began to appear in front of their eyes, and the air trembled with a cool gust. Something was waiting for them on the other side.... And then they heard a voice that clearly spoke their names.