It was a late, stormy evening when Lena, Kuba and Max entered the old building on Willow Street. The neighbourhood had long been shrouded in disrepute - an old, closed dentist's surgery around which rumours of strange experiments and missing documents circulated. The friends, avid members of the school's Detective Club, decided to see for themselves how much truth there was in the urban legends. The interior smelled of old mint and dust. The office looked as if its owner had only left for a moment - dental instruments lay scattered on the desk, an armchair upholstered in cracked blue leather stood against the wall, jars of strange contents piled on the shelves. Lena could not take her eyes off the cabinet marked with the warning: "Do not open! Research in progress." As Max pressed the torch button on his phone, the light reflected off something metal in the corner. Cuba walked closer and discovered a silvery toothbrush on the floor, which looked very different from the ones you can buy in a shop. Its handles were covered in intricate patterns and its bristles shone green. Lena carefully picked up the toothbrush, and at the same moment the door to the study slammed shut with a bang. At the same moment, a light flashed in the corner of the room. A hologram of the old dentist appeared on the wall and spoke in a mechanical voice: "Welcome, chosen ones. If you want to leave this place, you must find out what links oral hygiene to the secret of longevity. But beware - not every answer leads to freedom." The three friends looked at each other in disbelief as another hitherto hidden door flashed under their feet