Maks looked through the dirty glass at the courtyard. The tenement on Długa Street had stood here for over a hundred years, with its cracked walls, hidden courtyard and rusty gate that creaked with every movement. It always seemed to him that time passed differently in this place - as if echoes of the old years still floated between the bricks. It was Saturday, the beginning of the summer holidays. The sun had already set, but the heat of the day did not subside even for a moment. Lena, Max's younger sister, was sitting on the stairs with her nose in a book. When she heard her brother running down the stairs, she raised her head. - Are you going to go scouting? - She asked, lowering her voice. She was two years younger, but her curiosity about the world was second to none. - I think I have to. Those lights under the cellar again, did you see? - Maks nodded towards the unclosed door leading to the stairwell. For days, all the residents of the tenement had been whispering about the strange flashes seen at night at the end of the basement corridor. Nobody dared to check where they were coming from, but Maks and Lena always liked a challenge. With a rucksack full of torches and a supply of candy bars, they set off together towards the door. Before entering, they stopped. - Who do you think is hanging around down there? - Lena asked, adjusting her glasses. - Maybe someone has set up a hideout. Or... - Maks hesitated because he didn't want to scare his sister, but she just smiled. - Come on, let's check! They walked down slowly, the steps creaked under their feet. The cellar was cold and the air smelled of dust and old pipes. Some quiet humming came from far away. The closer they got to the end of the corridor, the clearer the light became, seeping from under the door into the long disused coal store. Maks carefully pressed the handle. The door gave way with a quiet rasp. Inside was something they had not expected at all: old cardboard boxes were spread out on the floor, with small coloured lamps flickering among them. In the middle sat someone who, at the sight of his siblings, rose abruptly. His face was hidden by a shadow, but something metal flashed in his hand. On the wall behind the figure was a strange drawing, as if created by a child - a maze of tangled lines and symbols that Max had never seen before. - Who are you? - chuckled the stranger in a low whisper. - Who sent you here? Lena squeezed her brother's shoulder, and Max's heart began to beat faster. It was just the two of them here, and a long, dark corridor piled up behind them. At the same moment, the lights flickered even more intensely, and a metallic clatter came from the depths of the storage room. Something that had hitherto looked like an ordinary brick had broken away from the wall, revealing a narrow tunnel plunged into darkness. Maks hesitated for a split second and then took a step forward, feeling the tension building up. What awaited them on the other side of this inconspicuous passage?