On Maple Street, under the big chestnut trees, stood a big yellow school. The pupils described it as the most ordinary in the world, but only until something happened that turned every school day upside down. Olive, Maks and Tom, three friends from class 5b, witnessed this change. It started on Tuesday morning when everyone, as usual, was waiting for the first bell. The sound, which usually resembled an old rooster after a cold, was like a signal to start another adventure. But on this day... silence. Pupils crowded the corridors, looked at their lesson plans and glanced shyly towards the portrait of the headmaster, as if he was about to explain an invisible riddle. Teachers waved their hands, trying to grasp the situation. - Maybe it's a malfunction? - whispered Maks to Olivia, but the girl shook her head. - The bell has no right to break, it's new! After the first lesson without the bell, the chaos grew. No one knew what time the lesson ended, and at break time teachers waved rulers to drown out the noise of the students. At the long break, the friends met behind the big corkboard, where Tom came in with a crazy theory. - Listen, that bell couldn't have just disappeared! Someone stole it or... - here he suspended his voice dramatically - hid it! They decided to start their own investigation. They started with room 12, where assemblies were always held, because that was where they had last seen the bell. When they went inside, there was something strange in the air - as if the smell of cocoa mixed with dust. In the middle of the hall, under the dais, they noticed something shiny. Suddenly they heard someone's footsteps in the corridor. The door slowly began to open....