The Stefan Żeromski School, although outwardly quite ordinary looking, hid more secrets within its old walls than the average pupil could possibly suspect. Marta, Jakub and Oskar were about to find out. It all started one rainy Tuesday. After lessons, Marta stayed late to revise a physics test. The school was almost empty that day and the corridors were full of echoing footsteps and creaking doors. As she left the classroom, she noticed a small brass key on the floor. It was decorated with strange markings and looked very old. Instead of taking it to the secretary's office, she put it quickly in her pocket. The next day, during the lunch break, they showed the key to Jakub and Oskar, who immediately picked up on the subject. - Maybe it's an old lock pick? - asked Oskar, turning the key in his fingers. - Maybe it opens something ... that they don't want us to see? - added Marta with a smile. At night, just before the school closed for the weekend, the three of them sneaked into the oldest wing of the building, where apparently no one had gone for years. The signs on the doors were worn down, but one door caught their attention: room 213. The small metal sign was covered in dust, and the lock was exactly the shape of their key. Oskar, with a pounding heart, slid the key into the lock. The door opened slowly with a quiet creak. Inside, there was semi-darkness. First they saw old desks and chairs, casting strange, elongated shadows on the walls. But then Martha noticed something else - there were notes on the blackboard, which had just started to glow with a greenish light.... At the same moment, they heard muffled footsteps and rustling behind them, as if someone was wiping the wall with their hand. Martha wanted to say something, but her voice caught in her throat. Was anyone else in this office? Was the key really just to open the door?