The bell rang to announce the end of the last lesson, but Veronika, Michal and Lena were not in a hurry to go home. The new wing of the school, where the never-ending renovation was in progress, attracted their curiosity like a magnet. Michal, in particular, couldn't stop thinking about the strange noise he heard there yesterday when the teacher asked him to bring in the chalk. The walls here were still unfinished, and pale light seeped through the dusty windows. The floor creaked underfoot and the smell of fresh paint mingled with the mustiness. Lena noticed that one of the tiles on the wall stood out slightly. They approached cautiously. - 'There's something here,' whispered Veronika, peering at a piece of metal under a loose tile. After a moment, they were all holding an old, rusty key in their hands. Michal tried to remember if there was an extra room in the renovation plans. However, he had heard nothing of the sort. Moving on, they came across a small white door hidden behind bags of cement. A plaque bore a barely legible inscription: "Sanctum Dentis". - It's Latin,' Lena whispered. - 'Something from the teeth... maybe 'temple of the tooth'? They exchanged communicative glances. The key fitted perfectly. Inside was almost complete darkness, but a narrow streak of light brought out of the gloom a row of old-fashioned barber's chairs, a large clock and what looked like a strange glass urn filled with hundreds of toothbrushes. Portraits hung on the walls and a mysterious box stood in the middle of the room. An illuminated sign appeared in the middle of the door: 'Without answering the Hygienist's question, you will go no further'. At the same moment, there was a rustling sound and a muffled voice from behind the seats: - Who dares to disturb the rest of the Guardian of the Smile? The teenagers froze in place, watching in disbelief as something began to emerge from the shadows....