Friday afternoon, the computer room at School Complex 17. The sun was sipping lazily through the blinds, and Lena stayed after school again. Not because she had to, but because she liked the quiet once everyone had left and she could finally hear her own thoughts. She strolled between the rows of computers, tapping her fingernail against the keyboards. It was then that she saw it for the first time. A number appeared in the air above the monitor - blue, slightly flickering, resembling a hologram. "3971" - she muttered under her breath, clenching her eyelids and opening them again. The number was still there, in the place where she had seen a void only moments ago. At first she thought it was the effect of fatigue or perhaps a new feature in the computer system. But after a while, the numbers began to appear over other things - over chairs, backpacks and even over her own hand as she tried to bring it closer to her eyes. Each number was different, pulsating, sometimes changing imperceptibly. Lena felt a shiver run down her spine. Had she just gone mad? The next few hours were like a rollercoaster ride. On the way home, the numbers also appeared on the street: over a cyclist, a road sign, even over a sleeping dog on a bench. She didn't ask anyone or mention it to her parents. It was only the next day, at school, that she told Cuba - her only friend, a fan of puzzles and science fiction - about everything. For a moment she thought he would laugh at her, but Kuba just nodded and asked her to describe the details. "Maybe it's some kind of code?" - he wondered aloud. He suggested experiments: Lena was to write down all the numbers she sees next to different people and objects. After just a few attempts, they discovered the first correlation - the numbers changed when someone was lying or upset. The school began to hum with rumours. Within a few days, something unusual happened - someone unlocked all the doors in the cellars. Parts were disappearing from musical instruments, and a strange message written in a cipher that no one could crack appeared in the maths teacher's office. Lena began to notice that next to some people the digits were going crazy: flashing, rapidly increasing or suddenly disappearing. One day, coming home with Kuba, they found a note on the library door with a mysterious code and the caption: "Do you know the truth? See if you can see it". Lena looked at the piece of paper and the numbers around it began to swirl as if they were dancing a tumultuous dance. She felt like she was about to faint. Cuba squeezed her hand, looking fearfully into her wide-open eyes. Then the library door suddenly opened - and there was a strange sound inside....