School No. 7 on Green Street looked very different after dark. The corridors were filled with the echo of their own footsteps and the fluorescent lights flickered, as if unsure if they still wanted to work. Veronica, Michal, Igor and Lena had never planned to stay at school after the final bell, but working on their physics project had drawn them in more than they had expected. When they finally reached for their backpacks, they realised that the door to the main hall was locked with a deadbolt. - Impossible, we were supposed to leave at seventeen! - Veronika pulled the handle once more, as if that would make a difference. Michal looked through the milky glass windows into the teachers' office, but everywhere was empty. Igor, somewhat frightened, chuckled: - Why don't we stay here until the morning? All in all... I've always wanted to see what the school looks like at night. Lena raised her eyebrows in a bid to defuse the tension: - I think it's your lucky day, Igor. The laughter was interrupted by a sound they could not ignore. A rustle came from deep in the corridor, as if someone was rummaging through cupboards or moving chairs. Veronika squeezed her physics book tighter. - I don't think we're alone here,' she whispered. They decided to see what was going on. They walked cautiously towards the physics lab, where the light was still on. The door was ajar. Through the crack they saw Professor Bialek, their physics teacher, trying to quietly pack some strange metal device into his backpack. - Professor? - Lena asked quietly, but he did not answer. At the same moment, the professor turned abruptly, looked at them with wide-open eyes, and then... disappeared. He literally became a blur in the air, leaving behind only a metallic smell and a strange electrical crackle. Everyone looked at each other, trying to understand what had just happened. Was it possible that Professor Bialek had really disappeared? And if so... what was the device he had taken with him? They felt a cold thrill of excitement and fear as they made their decision: they must discover the truth before the school doors open again at dawn....