At the end of June, the old school on Poplar Street looked very different from usual. Long shadows danced in the school corridors and the walls were covered with colourful drawings that seemed to move in the grey. All the pupils had already gone home for the holidays. All but three kids: Olga, Antek and Janek. - Did you see? That light flashed again! - whispered Olga, whose thick pigtails were trembling with emotion. Antek clung to the wall, glancing at the old corkboard. - Don't be afraid of anything. After all, it's probably just the reflection of a lamppost - he tried to give himself courage, although his heart was pounding faster and faster with every step. Janek swung open the library door, which creaked loudly, heralding new trouble. - I swear, I heard a voice. There's someone in there! That evening, all three of them decided to stay at school for the night. They wanted to see what really goes on in an empty building after hours. After all, there were stories circulating that things happened here after dark that no teacher had ever seen. They had prepared in advance as they should: torches, a backpack with sweets and a notebook to write down the most interesting observations. At first, everything seemed so ordinary. They strolled through the familiar corridors, whispering and laughing at their own jokes. But as the corridor clock struck midnight, everywhere became quiet and chilly. Suddenly, the light at the end of the corridor went out, even though the whole school should have been plunged into darkness by now. Olga squeezed Antek's arm tighter, and Janek leaned towards the stairs leading to the basement. - Can you hear the noise? - whispered Janek. Everyone's gazes turned to the padlocked cellar door, which... was now slightly ajar. - They were locked, after all! - groaned Antek. Something rattled quietly behind them, and a long, dark shadow flitted across the floor. Olga took a deep breath, gathered her remaining courage and took the first step towards the cellar. The boys followed her, feeling goosebumps on their arms. At that very moment they heard a strange, dragging sound, as if someone was moving something heavy very slowly across the concrete.... And then a quiet whisper came from the depths of the cellar. Neither Olga, Antek nor Janek knew yet what awaited them down there, but they knew one thing: they could no longer retreat...