The old High School No. 3 in the city centre looked quite ordinary on the outside - grey walls, chipped staircases, cracked glass in places. But for Julka, Victor and Sarah, this school had long been more than just a place of learning. It was like a labyrinth, where every day brought new surprises and the corridors had their own stories. One chilly October morning it all started with a strange incident. Julka was sitting on a bench in front of the school, looking at the wide-open front door. Victor, her neighbour from the cage next door, approached, brandishing his breakfast in a plastic box. - Did you hear that? The bell rang at six o'clock this morning! - He chuckled with concern. Sara approached them a moment later, correcting a storm of red hair. - 'My sister was on the school playground by then. She said no one was there, but the sound could be heard throughout the building. Intrigued, they decided to check the matter out during the long break. During the day, the school pulsated with life - students flitted from class to class, and there was the usual bustle in the corridors. However, at times when everyone was busy, something unusual could be seen: someone was whispering under the door of the closed room 12c, where the library had long been located, and the window on the second floor was always jammed when it rained. Victor, who always carried a torch and a small first aid kit with him (just in case), persuaded the girls to do a little reconnaissance in the afternoon. - Maybe we could find out who turns the bell on at such hours. They all gathered after school. The corridors were deserted and the light peeked through the colourful stained-glass windows in the stairwell. They walked carefully to the top floor, where no one had used the former physics office for years. - Did you know that no one has been coming in here for several years? - whispered Sara, looking back. In front of the study door, they came across an old sign that read: "Do not enter without permission!" Julka carefully pushed open the door, which creaked ominously, and they looked inside. The room greeted them with coolness and dust. A dusty old bell hung on the wall, with a bent cable leading to it. Victor moved first, the torchlight lighting their way. Suddenly, from somewhere below, the distinctive sound of the bell rang again, this time completely incongruous with the time of day. They froze for a moment, listening. Someone or something must have set it off. - It can't be... - whispered Julka. Suddenly, something rustled from behind a cupboard on the other side of the room. All three looked at each other, their hearts beating faster. Who would dare to approach first? And what will they find in a moment in a dark corner of the abandoned laboratory?