It was a sultry June evening when the last bell rang mournfully in the corridor of High School No. 7, leaving only echoes and shadows behind. Lessons had long since ended, but five high school graduates had just gathered in the chemistry room in the B wing. Gosia - the eternal optimist and self-proclaimed team captain. Marek, who could work out any cipher or logic puzzle. Natalia - a secretive dreamer with a camera always slung over her shoulder. Kuba, the cynical prankster, and Igor, fascinated by science to the extreme. They sat over a desk covered with reagents. Ostensibly, they were supposed to be conducting the final rehearsals for a science competition, but deep down they couldn't resist the excitement - they were alone in an entire wing of the school, the front door locked by the school caretaker, who explained with "the old protocol of closing the building after eight o'clock". The caretaker laughingly assured them that he would be back at exactly midnight. Half jokingly, half seriously, he threw in a departing remark: "Don't forget to leave everything in its place. And don't go to the cellar!" Gosia rolled her eyes. - They're probably storing out-of-date yoghurts from the buffet themselves in that cellar. - Or whole piles of notebooks lost by first-graders," added Marek. - 'Focus, we have to do it right,' interrupted Igor, dipping a glass pipette into the sample. The smell of chalk, old linoleum and something else, difficult to identify, hung in the air. Strange noises drifted through the corridors of the school - the creaking of old doors, the quiet reverberation of footsteps, although there was no one around but them. Natalia suddenly raised her head from above her notebook. - Did you hear that? As if... someone was walking down the hall next door. Kuba blinked and tried to make everyone laugh: - Maybe it was the headmaster who came to check who was stealing the chalk? But no one laughed. Gosia stood up. - 'All right. Let's check it out. After all, we're adults and we're not afraid of dark corridors. They gathered, taking torches from their phones with them. They walked through the empty corridor, passing bulletin boards with faded photos of competitions from the 1990s. The closer they got to the door to the adjoining hall, the quieter they began to talk. When they swung the door open, they saw that the light in the room was on by itself. There was an open notebook in the middle, and right next to it, a strange metal ball glinting in the lamplight. Suddenly, the door behind them slammed shut and there was a long, vibrating sound in the corridor - as if someone was scuffling something heavy on the floor. Gosia looked at the others, and Igor whispered: - It couldn't have been the wind... At the same moment there was a flash on the interactive whiteboard, and a blurry image....