On Friday afternoon, when most of the students were just thinking about the weekend, Lena, Kuba and Dawid met by the old dental clinic building. It had been abandoned for years, but there were rumours that something unusual was hidden inside - secret rooms that no one knows about. For the second-year high school students, such challenges were like a magnet: not only for the adrenaline rush, but also out of sheer, unexplained curiosity. Kuba, the bravest of the three, yanked a fibreboard from a rusted door. From the outside, the building looked completely dead, but Lena, who loved science and read up on medical trivia, noticed fresh shoe marks in the sand. Inside, there was semi-darkness and the smell of old chlorine. David shone his torch over the dusty educational badges still hanging on the walls: "Take care of your teeth!", "Regular brushing = healthy smile!". Their gaze was drawn to an old fire cabinet, behind which someone was clearly trying to hide something. After a moment's struggle with the lock, which almost fell apart in her hands, Lena pulled the handle and... to everyone's eyes appeared a narrow concrete tunnel leading deep into the building. In a whisper they rushed inside, trying not to make any noise. After a few minutes of walking in silence, during which they could smell the scent of chemicals more and more clearly, they reached a heavy metal door with the sign "Smile Laboratory. No admission." A pale light smouldered behind the door. They cautiously stepped inside and saw a room full of test tubes, flasks and mysterious machines, one of which was switched on and displayed the words on a monitor: "Caries vaccine - testing. To be injected: 3 doses'. Suddenly, something moved in the corner of the room. The heroes froze as a figure in a doctor's smock, with a mask over her face, emerged from behind a shelf. She looked at them penetratingly and quietly said: - You have come just in time. You must decide if you are ready to know the truth....