Charles had always claimed that school was a predictable place - bells, tests, endless repetition of the same subjects. However, his first day at Copernicus Grammar School was to seriously shake him of this belief. The red-brick building, with a turret over the entrance, looked like it had been transported from another era. The high ceilings and creaking floors heightened the atmosphere of mystery. It all started in chemistry class. Mrs Grzebisz, a teacher with a steely gaze, divided the class into teams and set the task: "Prepare an experiment on the topic of neutralisation reactions. You will find the reagents in the cupboards against the wall." Karol, not knowing anyone yet, joined the closest team - there were Lena, always a thoughtful literary girl, and Tymon, an amateur computer scientist known throughout the school. While searching for test tubes, Charles noticed something strange. One of the lockers, located in the furthest corner of the room, was locked with a rusty key. On its dark surface was a faded inscription: "Do not open". After the lesson, Charles couldn't help himself. He stayed a little longer on the pretext of correcting his notes and noticed the teacher hurriedly closing the classroom, but hiding the locker key in her jacket pocket. When the lights went out and the corridor fell silent, Charles returned to the chemistry door. To his own surprise, he noticed that a faint purple light was seeping through a crack under the door. Then he felt a gentle touch on his shoulder. He turned slowly, ready to run away - but saw Lena and Tymon, who were clearly here for the same reason. "Do you see it too?" - Lena whispered. Before they could do anything, the door swung open with a quiet creak. It was completely dark inside, except for the mesmerising glow coming from the mysterious cupboard. Charles felt a strange tingling in his fingers. Lena took a step forward, Tymon held his breath. Suddenly there was a deafening bang from inside the cupboard and something began to slowly emerge from behind the ajar door....