Not every day at school starts with a discovery that can change everything. Lena, a thirteen-year-old with a penchant for science and endless questions, was sitting with Igor, her best friend, in the biology room. It was Tuesday and the inquisitive teacher, Mrs Krzewicka, had just brought in a new microscope. Its streamlined shapes and shiny knobs caught the eye of all the students. - Do you want to see what happens if we look at something unusual? - whispered Igor, handing Lena a small glass plate with a drop of saliva on it. Lena shrugged her shoulders, but their curiosity won out. They leaned in together and looked through the lens. At the same moment, they felt a strange tingling sensation - as if the air had thickened and pulsed. Before they had time to protest, everything swirled. When Lena opened her eyes again, she was standing on a colourful shiny bridge made of... enamel? Beside her, with an expression of bewilderment, appeared Igor. They ran their eyes down the bridge. All around them stretched an incredible city - the Kingdom of Teeth. Huge whitewashed gates surrounded the edifices, and bizarre creatures roamed the streets - filigree bacteria, armed with mini-brushes, patrolled the pavements. Against the wall was a notice board with the words: "Caution: caries gangs on the loose!". - This... are we inside the mouth? - mouthed Lena. Igor looked around in awe. - It looks like we are. But... look! From within a dark furrow in one of the molars, something began to move. Slowly at first, then faster and faster, a flock of black, rustling creatures glided towards them, leaving a brown streak behind. Lena squeezed Igor's hand. - 'We need to hide,' she whispered, leading him behind a nearby mural of toothpaste. Behind the mural, others were already waiting - two strangely familiar Heroes of Hygiene: Captain Brush and Commander Floss. They looked at Lena and Igor with seriousness. - 'We need your help,' said Captain Brush. - The decaying gangs are preparing something the Kingdom of Teeth has never seen before. You must come with us... They barely had time to ask the question when the ground began to tremble slightly, and louder and louder sounds came from the depths of the tunnel....