Primary School No. 12 looked like any other school, but only to those who did not know its secrets. Tola, a thirteen-year-old with a huge imagination, and Sebastian, a fourteen-year-old comic book lover, never believed the stories about strange sounds coming from the school's underground - until they heard them themselves. It was a Thursday evening when they were walking back to the cloakroom from the library. It was then, by an old bench in the basement, that they came across something unusual - between the bricks shone a silvery handle that had never been there before. Of course, curiosity prevailed. The door creaked open and to their eyes appeared a dark corridor smelling of mint and.... toothpaste? Without hesitating for long, they took the torches from their backpacks and went inside. The corridor led lower and lower, and the walls were decorated with pictures of strange figures - toothbrushes and teeth in knightly armour. There were mysterious whispers coming from there: "Brush thoroughly.... floss regularly...". Finally, they reached a chamber that had been transformed into a shimmering laboratory. In the middle stood a huge throne made of giant white cubes.... were they teeth? A figure emerged from the darkness, dressed in a white apron, with a toothpaste-like cloak spread over its back. The figure smiled broadly, revealing teeth that were perfectly even and shining like pearls. - 'Welcome to the Guardian of the Smile,' she said in a low voice. - You have been chosen to carry out a most important mission.... But before you find out what it is, you must pass a test of courage. The first task: defeat the decay bacteria that have sneaked into my kingdom! Tola and Sebastian looked at each other in surprise. Mechanical toothbrushes slid out of the walls and strange black creatures resembling dirty food remains began to crawl out of the nooks and crannies. Would they manage the task and discover the secret of the underground smile?