The city library has never been a place of exciting adventures for Kuba. Everything changed on that cloudy Thursday, when he, Ola, Kamil and Natalia decided to do their homework among the shelves full of books. Thinking that they would get through their assignments quickly, they did not expect that this day would change their lives. By chance, they came across an old bookshelf full of maps. When Kamil wanted to show the rest a map from over a hundred years ago, he noticed that the entire section had disappeared. Kuba tried to ask the lady librarian, but she only gave them a slightly too meaningful look and whispered: "Not everything should be so easily accessible." In the evening, after the library had already closed, the four of them returned, trying to find an explanation for the mysterious disappearance. Natalia noticed a small door in the corner, barely visible under the drooping wallpaper. The door led to a winding stone staircase down - the basement of the library. With each step, they could hear the murmur of water and the muffled echoes of someone's voice more and more clearly. As they reached the bottom, the torchlight fell on a huge hand-painted plan of the city - the same one they were looking for! But they were not alone. Somewhere out of the gloom, someone's eyes flashed, and the question hung in the air: who else was looking for the hidden maps and what secrets did the underground hold? Silence fell for the moment, which was suddenly broken by the quiet sound of shifting stones. Everything indicated that the real adventure was just beginning....