Lena was never fond of visits to the school hygienist. She associated them with boring chats about brushing teeth and threatening brochures about tooth decay. However, since the new dentist, Dr Czyszczak, arrived at the school, everything seemed ... a little weirder. He was a middle-aged man with an unusual sense of humour, always with pockets stuffed with coloured toothpaste, which he handed out right and left. He had turned the surgery he took over from his pensioner Maria into something of a laboratory: not only mirrors and a dentist's chair, but also mysterious apparatus, test tubes and even a small pot with a carnivorous plant he called "Plateusia". One afternoon, Lena stayed after lessons to collect her dental certificate - a document essential for the school trip. As she slipped into the office, she noticed Dr Czyszczak talking to someone on the phone in a hushed voice. His words sounded quiet, almost conspiratorial: - Yes, the sample is ready. If it's successful, the kids will never be afraid of brushing again.... I just need to check one thing this evening. Lena froze. What could be going on behind the closed doors of the school dentist's office? Was the doctor actually creating something that would change their bathroom routine? Or was this the beginning of some strange experiment? The girl decided to wait until later and try to find out what secret was hiding among the toothbrushes, pastes and tubes.... From hiding, she watched as the doctor set a mysterious illuminated vial on the countertop, which shone with an intense blue light. At the same moment, a violent knocking sounded outside the window, as if someone was trying to get inside. Lena held her breath, feeling her heart rumbling in her chest. Should she reveal herself? Should she run away? Or should she... enter the study and try to solve the mystery of Dr Czyszczak?