At the Medical University in the middle of the big city, right next to the modern library, there was an unusual place: Professor Swieżyzub's laboratory. Although well into his teens, Professor Swieżyzub was known for his unconventional ideas and his ongoing passion for oral hygiene research. His laboratory was full of strange devices, vials of coloured liquids and models of teeth bigger than basketballs. One evening, when the city was shrouded in an intense June twilight, four dental students - Lena, Maciek, Basia and Filip - decided to take advantage of a unique opportunity. For the professor had offered them to stay after hours and help him with a new experiment on bacteria in the mouth. The laboratory at night was completely different from the daytime. The shadows cast by the microscope lamps danced on the walls and the distinctive smell of toothpaste was in the air. The students behaved casually at first, joking among themselves about laughing teeth and brushing each other. However, the atmosphere quickly thickened when Professor Svyazhub closed the heavy door behind them and solemnly announced: - Tonight we will discover something that no one has seen before. I count on your courage and attentiveness. Intrigued, they all set to work. Lena prepared the slides for the microscope, Maciek poured the liquids into the measuring cups, Basia took notes on the professor's observations and Filip controlled the complicated computer connected to the special camera. The deeper they got into the research, the weirder things started to happen. First, unusual sounds began to come from Filip's computer, reminiscent of the quiet rustling of a toothbrush. Maciek noticed that one of the vials had turned an intense blue, although it should have remained transparent. Lena, looking into the microscope, began to see previously unknown life forms that flashed under the light like miniature lanterns. Basia struggled to keep up with recording the results, as she increasingly had to wipe the steam off the glass that appeared from inside the room. Finally, the clock struck midnight. Then something happened that made everyone's heartbeat accelerate. A colourful image of a strange, busy world appeared on the big computer screen. Images from the microscope merged into a single animation, showing the life of oral bacteria in real time. The microscopic figures began to move, making quiet sounds as if they were trying to say something. Everyone stared at the screen as one of the bacteria turned to the camera and its glowing 'eyes' looked straight at them. For a moment there was absolute silence in the laboratory. And then, completely unexpectedly, the lab door slowly began to open, although Professor Swiezub had the only key with him....