It was a warm June evening. The city was already sinking into twilight, and five friends, Lena, Krzysiek, Alicja, Borys and Maks, had gathered in front of the Kopernik Secondary School. They loved urban riddle games - especially those embodying urban legends. This time, the task fell to them: to unravel the mystery of the 'White Mask' - a mysterious guardian of oral hygiene who allegedly haunts an abandoned dentist's office on the outskirts of the city. Lena, the group's guide and a fan of detective stories, held up a map drawn by the game's organiser. "The last clue leads straight to the back of the old surgery. Have you heard all those stories about braces slipping off teeth after dark?" she chuckled with a glint in her eye. "Stop it, Lena, it sounds like a horror movie for primary school kids," laughed Christopher, although he didn't stop looking around nervously. The old office looked like it had been taken alive out of a horror film. The rusty sign "Dr Anna Mint's office" swayed in the wind. Through the dusty windows, one could see rows of armchairs and a giant poster with a smiling tooth that seemed downright mocking in the darkness. Alice pressed the door handle - it surprisingly gave way. The smell of old fluoride and cotton wool hit their nostrils. The light of torches glided across cabinets full of toothbrushes, tubes of toothpaste and packets of floss. "Look!" - cried Boris suddenly. On one of the armchairs lay a white mask on which, instead of eyes, two huge, shiny incisors were drawn. Beside it someone had arranged a letter: "Only those who take care of their teeth will leave here with a broad smile. The rest...?" At that moment, the sound of instruments being moved and something that resembled a quiet giggle came from the depths of the office. Maks held his breath and the others instinctively backed away from the masked chair. In the glare of the torches, the strange writing on the wall glowed: 'WHAT YOU HIDE UNDER YOUR LANGUAGE WILL BE DISCOVERED TONIGHT'. Will they be brave enough to find out what hides in the locked cupboard at the end of the corridor? Or is the mysterious White Mask already behind their backs?