The hot Friday evening seemed to last forever. At Tadeusz Banach High School, the wind was playing with the dusty curtains and the corridors were empty as most of the students had already rushed off for their holidays. Three friends - Lena, Igor and Bartek - stayed at school a little longer, finishing a physics project in room 213. Lena stretched, finally putting down her ruler and notebook on the windowsill. - 'We're done,' she announced with a smile. Bartek looked at his watch. - 'We should have been home a long time ago, they're about to close school. On the way to the cloakroom Igor stopped suddenly. - Have you ever seen this door? - He pointed to the narrow old door between the history room and the carpentry shop. Lena furrowed her eyebrows. - 'Never. I thought there was just a wall there. Bartek tried to pull the handle. The door was locked with an old, heavy lock, and a note was taped above the handle: "Do not enter! Renovation work." The tape had long since fallen off the card, the letters were faded, and the whole thing looked as if it had survived in this place for several years. Igor lowered his voice. - And remember what Kuba from the third B said? That there are places in this school that you are not allowed to look in.... Lena rolled her eyes, but her heart beat faster. - It's just a rumour. - Maybe... - Bartek looked at the door more carefully. - But check this out! By the doorframe someone had scratched out the letter 'S', and above it several numbers could be read: "4-1-7". Lena leaned closer. - Maybe it was some kind of cipher? At that moment, there was a rustling sound from the depths of the empty corridor, followed by a quiet clatter - as if someone had dropped something on the floor. All three of them froze. When Lena turned around, she was sure that a shadow had flashed at the end of the corridor. The sound repeated itself, closer this time. Bartek moved slowly away from the door, squeezing the strap of his backpack tighter. - Anyone else left here...? Igor shook his head. - That's impossible. The usher was already closing the school entrance. Lena took a step towards the mysterious door. - Maybe we should take a look here... Then a loud knocking sounded from inside. Five short beats. Silence. Another, quieter knock. Everyone held their breath. In the corridor, the lights began to flash.