A Friday afternoon at the high school always had something magical about it. The long corridors seemed quieter than usual, and the computer room was filled only with the clatter of keyboards and the hum of computers. In this apparent monotony, Lena, Filip and Basia were searching through lockers for lost items after programming class. It was Basia who first spotted an old brown notebook wedged deep between yellowed textbooks. On the cover, just under a layer of dust, was an old-fashioned drawing of a key. Lena glanced at it curiously. - Maybe it's some diary of someone from previous years? - she whispered, and Filip was already looking for his phone to take a picture. When they opened the notebook, they saw not only rows of incomprehensible symbols and equations, but also the names of teachers who.... should not have been teaching yet at that time, at least according to the dates written in the margins. The pages were full of strange codes and clues - as if someone had left them a riddle. Every few pages they found illustrations of parts of their school, some that seemed familiar, others that looked like they came from a completely different building. - 'This looks like a puzzle,' Philip muttered, tracing the strange sequence of numbers and letters with his finger. - 'Maybe someone was trying to tell us something? Suddenly, the light in the room flicked on and off for a few seconds. When it came back on, a message appeared on the monitor of the nearest computer: "If you are reading this note, the time has already started". Everyone froze. Something clicked quietly in the server cabinet, which was still locked, and dust swirled in the air. Lena looked at her friends with growing anxiety and excitement. - 'I don't think we have any other choice, we have to check it out,' she said. Philip carefully closed the notebook and they moved slowly towards the door, unable to stop thinking about the question that was now hanging over their heads: What is really behind the code in the old notebook?