When Gabrysia returned from school, she did not expect that another day at high school would be the start of the most extraordinary adventure of her life. Nothing exciting had ever happened in Jaworstok. Until now. It all started in the school cloakroom, just before a chemistry lesson, when Adam, Gabrysia's best friend, approached her with a strange look on his face and whispered: - Have you heard about this new case at the dentist? Gabrysia looked at him with slight concern. - What do you mean? - She asked. Adam glanced over his shoulder, as if checking to see if anyone was eavesdropping on them. - 'There's talk in biology class that someone is breaking into the hygienist's office at night. And that toothbrushes are disappearing. But that's not all! - he added, raising his eyebrows. - Apparently, suspicious post-it notes on the cabinets have started to appear. With some strange instructions like "Every day, twice!" or "Floss is power, not weakness!" Jacek, known for his huge imagination and love of challenges, joined the conversation: - And I heard that the director wants to close the hygiene room! Apparently, since this is happening, more and more students are complaining about .... well, such an unpleasant smell in the school. Gabrysia mused. It was indeed something unusual. After school, the three of them met in the school library, at a desk under the window. They decided to start their own club: the Night Brush Club. They each brought their notes. They collected stories about missing toothbrushes, about cards on lockers and gossip from different classes. There were more and more puzzles in front of them: Who leaves strange clues? Why are the toothbrushes disappearing? And is it possible that any of them are suspects? As they looked through more clues, Gabrysia noticed a note under her textbook that read: "Check the crack in the wall by the chemistry lab. That's where the answer lies." Everything happened quickly. They went quietly out into the corridor, trying not to let anyone notice them. There really was a small crack in the wall by the chemistry lab, in the half shadow under the windowsill. Adam slipped his hand in there and.... He felt something cold and sticky under his fingers. He pulled out a tiny metal casket. When they opened it together, inside they found something that froze the blood in their veins - and completely changed their perspective on daily oral hygiene....