On a dreary, rainy Wednesday, when most of the students were going straight home from school, Lena, Maks and Bartek decided to do something different. They had long been fascinated by the old, abandoned building around the corner of their street - once a dentist's surgery, now mired in darkness and abandoned memories. It was said that someone had abandoned it in a hurry, leaving tools, posters with beaming smiles and.... something else. Lena is the first to cross the threshold. The interior looked exactly as if time had stopped. The smell of old toothpaste was everywhere. Maks - always curious - noticed that one of the toothbrushes on display was strangely larger than the others and had a serial number engraved on it, as if it were part of some secret project. Bartek felt a shudder when they suddenly heard a quiet sound - as if someone was brushing their teeth in the further room. Following this sound, they discovered a door leading to the basement. On the stairs they found an abandoned notebook with written formulas and drawings of bacteria. "Looks like experiments with toothpaste," Lena whispered. In the basement, surrounded by semi-darkness, they came upon a large glass display case. Inside it was the TA toothbrush - the larger one. But it was strangely shiny, and its bristles seemed to move slightly. At the top of the display case stuck a badge: 'Prototype: Tooth Protector 6000 - Do not open!". Maks was already about to touch the glass when a shadow emerged from the darkness. Someone, or something, was lurking next to a cabinet of old dental floss. The sound of brushing returned - this time just behind their backs....