In the middle of a small town stands a red-brick library, whose cellars are shrouded in legend. No one remembered anymore exactly who built it or why there was always a slight mist in its cellars. For twelve-year-old Zosia, her younger brother Frank and their friend Kuba, the library was a place full of surprises. One Friday afternoon, when the rain was drumming against the windows of the reading room, the children decided to see if it was true that something unusual was hiding under the stairs leading to the cellar. Kuba, brave as ever, was the first to jump into the darkness, and Zosia and Frank refused to back down. They found an old padlock, which - to their surprise - opened when Zosia accidentally touched it with her old locket. The door creaked open and a long, narrow corridor lit up with glowing blue stones appeared before them. Portraits of the town's former inhabitants hung on the walls, and at the end of the corridor a door was covered with strange signs. The children felt shivers down their spines, but curiosity was stronger than fear. As they came closer, their eyes were drawn to the inscription: "Mystery awaits those who are not afraid of questions". The door slowly began to open and from behind it came a mysterious whisper, as if someone had been waiting for visitors for hundreds of years. Zosia, Kuba and Franek stood up as if startled, and an unfamiliar light shimmered behind the barely ajar door. What could be hiding on the other side?