When Staś crossed the threshold of Maria Skłodowska-Curie High School, all he felt was a tightening in his stomach. He had just moved to the city and knew no one here. His mum used to say that a new beginning was a chance for unusual adventures, but for the time being he only noticed the curious glances and murmur of conversation in the corridors. The school was old and majestic - the high ceiling of the auditorium boasted an ornate clock, and the narrow corridors hid hundreds of locked doors. Staś walked with his head lowered until he bumped into someone so hard that he almost fell over onto his backpack. - 'Careful, newbie,' laughed the girl, extending her hand to him. - I'm Lena. I got lost here once too. A moment later, they were already walking to class together, and Lena was explaining where it was better not to sit down, because the March sunshine was blinding for an hour and a half. Staś felt a little better. After lessons, Lena suggested a quick tour of the school. She led him through the labyrinth of corridors until they found themselves under the aforementioned clock. That's when Staś noticed something strange - a small metal door just below the dial, barely visible from behind the plinth. - Look - he whispered. - Has it always been here? Lena looked intrigued. - 'I never paid any attention. Probably no one has opened it for years. Suddenly they heard a quiet clang and... the door swung open slightly, as if someone from inside was trying to open it. A barely audible creak came from inside them and a piece of yellowed paper appeared. - Pull it out! - whispered Lena, looking at Staś with wide eyes. Staś slowly slid his hand inside and felt the cool paper under his fingers. When he pulled out the paper, they saw a strange drawing on it: a diagram of the school with a cross marked on it in a place where no known door or corridor led. - What is this? - groaned Lena. - 'Do you think it's... a map? Then two quick thumps of metal against metal came from inside the clock. After a moment of silence, they heard slow footsteps approaching the hall from the side of the empty wing of the school....