On Monday morning, the school was bustling with activity, even though most of the pupils were struggling to roll out of bed. The corridor echoed with the buzz of conversation, the clatter of trainers and cheers as long-lost friends bumped into each other right by the drinks machine. It was the start of a new term. Juliusz Słowacki High School looked as usual: old brick walls, narrow windows and grey, sky-drawn corridors. But something was different that day. When Lena, Maks and Bartek entered the biology classroom, they immediately noticed that someone had written a strange, illegible sign on the blackboard - it resembled a spiral with three diagonal lines. "Look, I don't think it's from Mrs Wisniewska," muttered Maks, glancing uncertainly at Lenka. The girl shrugged her shoulders. Bartek came closer, trying to rub off the symbol, but the chalk wouldn't come off, as if it was embedded in the surface. The lessons dragged on at their own pace until the lunch break. When the school bell sounded, this time something went wrong. Instead of the usual triple signal, there was a low, dragging sound, and the lights throughout the building dimmed slightly. The students turned to listen. "It's probably a short circuit," chuckled someone from a neighbouring bench, but then Lena noticed that the symbol on the blackboard lit up for a second with a pale blue light. "Oi, did you see that?" she asked in a whisper, pointing at the board. Bartek pulled out his phone to take a picture, but his mobile screen suddenly went blank. Maks tried to turn the door knob - it was locked. There was a rustle outside, as if someone was pacing the corridor. At that very moment, the lights flicked on again, and a quiet whisper came from the loudspeaker, which no one seemed to hear except them: "It's time to uncover the truth...". The friends looked at each other, feeling their hearts begin to beat faster. What was really behind the puzzling bell and the mysterious sign on the blackboard?