It was a quiet Monday, and the corridor at the Maria Skłodowska-Curie School was surprisingly empty. Karolina was walking alone towards the biology lab when suddenly something rustled under the door of the school bathroom. She pulled the headphones out of her ears and looked cautiously around the corner. There, kneeling on the cold tiles, was Kaj - the school's greatest expert on strange things. "What are you doing?" - whispered Karolina, glancing at the shiny object in his hand. Kaj raised his head. "I found... a strange key. It looks like a pinion from a big toothbrush! And look - here, in the wall, there's a little crack, like... like some kind of lock." Karolina felt a thrill of excitement. The school was old, and there were all sorts of legends about it - about ghosts, hidden hiding places, and - as the janitor Sophie claimed - about underground corridors. But none of these stories involved bathrooms. And certainly not about magic keys. "Let's try to put it in," she whispered, peering into the depths of the empty corridor. Kaj slipped the key into the slot, and then the floor rattled quietly, two tiles sliding apart, revealing a small descent bathed in pale blue light. Karolina looked at Kaj. "Shall we go?" - She asked, and he shrugged his shoulders, and a moment later they were underground. They walked down a dark, damp corridor until suddenly the walls lit up with fluorescent inscriptions: "Kingdom of Teeth. Let those who understand the power of cleaning enter!". They passed through a passage strewn with bizarre, brushed lianas, when Kaj tripped over a a huge molar lying on the ground. Suddenly, a being emerged from the darkness, resembling a mixture of toothpaste and ghost, with eyes like sparkling crystals. "Who are you? And do you know that the Kingdom is threatened by the Shadow Stain, which will erase the smiles of all the inhabitants?" - she asked. Karolina squeezed Kaj's hand, and their shadows danced in the faint light. Then a deafening bang sounded. The whole subterranean realm trembled, and from around the corner emerged a a great black silhouette.