Whenever it rained, the old building on the outskirts of Kwiatow resembled something out of a science fiction film. Greenish light reflected off the dirty windows of the former dentistry laboratory that once belonged to Dr Mandible. Rumours circulated around town about strange noises coming from inside and fluorescent streaks seen at night. In theory, everyone preferred to steer clear of the place, but for Stasio, Marta and Igor, it was a dream location for their "City Explorer Patrol" pack to meet. Staś was a bright fourteen-year-old obsessed with puzzles. Marta loved science - mainly biology and chemistry, and Igor considered himself a technical genius who could fix any gadget. That evening, armed with torches, notepads and one apple for the three of them, they sneaked under the wet branches towards the main door of the laboratory. There was a strange smell of mint in the air. - Are you all ready? - asked Staś, checking that Igor's dictaphone was working. - 'Sure,' nodded Marta, correcting her backpack full of samples from her room waiting to be described. As soon as they opened the door, they were greeted by a chill and a quiet hum. The room looked as if someone had interrupted work mid-day: open notebooks, empty tea glasses, yellowed posters with drawings of toothbrushes and scattered dental instruments. On the walls hung sketches of strange creatures resembling bacteria and viruses with smiles and toothbrushes instead of hands. Suddenly Igor stopped abruptly. - Look at this! - he whispered. Just beneath the windowsill, among the broken test tubes, lay a large, closed container with the words 'Fluoride 2.0 - testing continues'. Next to it was a drenched note: "Caution: activates on contact with inadequately washed teeth". - Oh no, I think that's... live bacteria? - Martha looked up fascinated. As they lifted the container, a piece of mouldy bread roll fell off the countertop. A luminous cloud shot out from inside, and coloured lights lit up in the laboratory, revealing something the three friends had not expected at all. Behind the glass window, in the dark part of the lab, someone or something was just starting to move. The figure's teeth gleamed like silver, and its gaze followed their every move. Then a strange, quiet voice rang out: - Who dared to disturb my patrol? The friends froze, not yet knowing whether they should run away or perhaps stay and help.