School number 11 on the Green Valley housing estate was nothing special - at least until that day. Lena and Igor, 13-year-old friends from the same class, were known for their constant search for strange stories and unobvious places. After lessons, instead of going home, they decided to check out a school legend: supposedly there is something in the old bathroom in the basement that no one has discovered yet. Pressing herself against the wall next to the forgotten pipe, Lena felt a small indentation. She pressed on it. It creaked. The tiles moved away with a quiet clatter, revealing a narrow stone corridor. Igor was stunned, but Lena was already taking the torch out of her phone. - You're not thinking of going in there, are you? - whispered Igor, but seeing his friend's gaze, he had no choice. They descended the steep steps. The humid air smelled of mint and.... strawberry? A strange popping could be heard. Suddenly, the light of the phone reflected off the glittering dome - they were in a huge cave! On stone benches sat miniature figures in white aprons, with tools resembling toothbrushes and dental mirrors. Above their heads floated clouds filled with drawings of teeth and coloured toothpastes. Lena and Igor hid behind a pillar. They heard a serious voice: - Guardians of the Pearl Smile, we must act quickly! Prodigy and his Band of Stone are returning. If we do not defend the Gates of White, our entire underground metropolis will crumble! Igor's heart beat faster. Lena looked at him with a mixture of fear and excitement. Something moved in the shadows at the other end of the cave and....