When the final bell rang to announce the end of Monday's maths lesson, there was little indication that anything could happen today. Class 3C slowly began to gather to leave. Susanna was packing up her notes, Adam was laughing at Bartek's joke and Julia, as thoughtful as ever, looked out of the window at the sunlit school playground. The teacher was the first to leave the classroom - he had to take an urgent call from the secretary's office. As the pupils approached the door, however, they realised that it could not be opened. The handle did not move. Bartek, wanting to impress the class, pulled harder, but the door did not budge even a millimetre. Comments quickly followed: "Maybe something is jammed?", "Someone locked us in for a joke?". The atmosphere became more tense by the minute. Julia tried to call the tutor, but her phone suddenly lost range. Susanna noticed that even through the window, the sounds from the school corridor could not be heard - as if the whole world had disappeared, leaving them alone. Adam started joking about ghosts, but even he was no longer laughing. Then, on the blackboard, between the mathematical equations, a strange, bluish writing appeared: "Find the object that isn't there before it's too late". Bartek jumped up, Susanna grabbed Julia's hand and Adam tried to wipe off the writing, but the blackboard was as if frozen. No one knew what it meant - was it a joke or was someone trying to warn them? An old radio began to buzz in the middle of the classroom, though it never worked, and a quiet, anxious whisper came from behind the closed door: "Time is running out..."