Until a few months ago, Lena could have sworn that nothing more exciting would happen in her 16-year-old life than a solo train trip to a neighbouring town. However, everything changed when, while skateboarding with Philip on a June afternoon, they stumbled upon a rusty trapdoor hidden behind the overgrown bushes of a former industrial park. Everyone knew that this park was a place to be afraid to venture into at night - both because of stories about homeless people and through urban legends about strange lights underground. But Lena was curious and Philip was impatient. They both had something else going on: a serious problem with newly weaned sweet cola and a mass of cavities after the last few months of quarantine. There was a silent ranking going on in their class: who would spend more on the dentist by the end of the year. That's when Philip accidentally kicked the lid and a metallic clatter went through their backs. At first they thought it was just another piece of junk. But when Lena looked closer, she saw a logo embossed on the metal that resembled a wide smile and a toothbrush. - 'I think that's the logo of that old toothpaste factory that went bankrupt in our parents' time, isn't it? - she chuckled, trying to hide her excitement. Philip took a torch from his pocket with his phone and pointed the light at the lock. Luckily, or unluckily for them, the flap was not locked. The hinges squeaked quietly as they swung it open and peeked into the dark, cool tunnel. - 'You first,' Lena whispered, feeling her heart beat faster. They went in. The smell of damp mixed with some strangely familiar scent of mint. They walked carefully between abandoned machines, bags of strange powder and smiling posters. Suddenly, a quiet, barely audible sound caught their attention: as if some machine was running somewhere further down the line. - 'I thought nothing had been going on here for years,' whispered Philip. They moved closer, passing an old production line. After a few minutes they came to a steel door, behind which a pale light was seeping from under the threshold. A note hung on the door: 'Entrance only for those who know the true power of clean teeth.' Lena felt a gentle tingling in the back of her neck. Should they go in? Who else could be on the other side? Or would what they find change not only their approach to oral hygiene, but their whole lives? At that moment, they suddenly heard quiet footsteps behind them....