Gabriela, Wiktor and Lena never expected that a simple lesson on oral hygiene would turn into the biggest adventure of their teenage lives. When the final bell rang and the rest of the class ran out of the biology room, they were stopped by Mrs Sikora. The old lady was enthusiastically explaining to them the importance of daily dental care, but at the same time she seemed as if she was thinking. Finally, she handed them an inconspicuous key and whispered: - I would like you to do me a favour. In the old laboratory, at the back of the school, there is a cupboard marked 'Project Fluorite'. Open it and bring me the green box. Just be careful," she added with a smile. The teenagers looked at each other with growing excitement. This was something different from the usual homework! In the evening, sneaking through the quiet school corridors, they felt like characters in a film. Gabrysia carried a torch, Lena couldn't help but giggle nervously, and Victor pretended to be brave, although he looked back every now and then. The old laboratory was completely different from the bright, modern classrooms. It smelled of dust and something indefinable, as if sweet. Vials and glass tubes glittered in the gloom. Somewhere, deep beneath the cabinets, a quiet humming could be heard. - There it is! - cried Lena, pointing to a metal cabinet with a faded inscription. Victor solemnly slid the key into the lock and turned it. The lock squeaked and the door swung open with a groan. Inside, on a spongy shelf, lay a green box bearing the symbol of a shiny tooth. As Lena pulled it out carefully, suddenly strange mechanical sounds began to come from a machine standing in the corner of the laboratory. After a moment, completely unexpectedly, a bluish light flashed from inside a distant corner and spread across the room, revealing... And suddenly all three of them froze, having heard something that could not be explained by anything logical. What or who was hiding on the other side of the light?