Denton Academy has always been a place where you didn't just learn biology or maths, but especially paid attention to... your teeth. The entire prestigious high school, located on a hilltop near the town of Wolf Meadow, was known for one thing: almost all graduates had perfect smiles. It was hard to meet a dentist who hadn't heard of Denton. The main characters were Lena, Maks and Igor - three friends who had just started their third year of school. Lena liked to think logically, Maks was interested in science and liked to experiment, and Igor had an unusual sense of humour and a talent for drawing cartoons (mostly about monsters with teeth that resembled power saws). It all started one ordinary Monday when Lena discovered that someone had hung a poster on the notice board: "Anyone who doesn't take care of their teeth is in danger of a BLACK STONE". The poster was cut out of a newspaper and depicted a black, shiny lump that looked like materialised tooth decay. - 'Someone's making a joke,' muttered Maks, looking at the poster. - Or trying to scare us - added Igor, looking at the students around them. That same day in the school bathroom, Lena noticed that there was... strange toothpaste lying in the sink. The tube had no label, it was metallic blue and the cap bore a symbol she had never seen before: two intertwined fangs surrounded by a swirling ring. During break, her friends decided to go back for the tube, but... it had disappeared. Notecards with strange slogans began to appear in the lockers of several students: "Do you wash your tongue?", "Do you floss or are you scared?", "Watch out for the stone". No one knew where they came from. Things started to change - more pupils with strange grey discolourations on their teeth appeared in school. The chemistry teacher, Mrs Brzozowska, reportedly received a warning from an unknown sender. In biology lessons, there were more and more questions about bacteria, plaque and methods of cleaning interdental spaces. Soon Lena, Maks and Igor discovered that the strange toothpaste was just the start of something bigger. At night, someone would sneak into the bathrooms and leave black marks on the mirror. One morning, Lena found a small, shiny pebble on her toothbrush, while Maks saw a slightly slimy, dark stain in his mouthwash cup. They decided to take action. Igor designed leaflets warning of the mysterious paste, Lena collected samples and documented the symptoms, and Maks tried to discover the composition of the unusual substance using the school laboratory. They collected all the clues they could find until they came across a locked door to a small, old room under the stairs. According to legend, this is where the hygienist's office used to be, but no one had looked there for years. With bated breath, Lena pulled out a key she had recently found in the library, hidden between the pages of an old dentistry textbook. She slowly slid it into the rusty padlock, while Maks and Igor watched to see if anyone was following them. The lock squeaked and the door swung open with a slight slam. Inside awaited them a sight they had not expected....