Primary School No. 15 looked ordinary only from the outside. No one suspected that there was something hidden under its floors that even the bravest pupils never dreamed of. Kuba, Lena and Maks had been friends since first grade, but since fifth grade began, their lives have taken an incredible turn. One autumn afternoon, when everyone was already going home, the three friends stayed on the pitch to finish a game of ball. As Maks caught the ball, he noticed a strange grid hidden under dried leaves by the old school wall. - 'Oi, look, what's that? - cried Maks. Kuba rushed to the grille first and was already trying to pick it up. To everyone's surprise, the grille did not move at all - until Lena touched it with her little finger. Then the grille rose up as if it were made of paper. - But you have strength! - exclaimed Kuba, looking at Lena with wide eyes. Lena was abashed, but she herself didn't know how she had done it. The three of them looked inside. Under the grille there were steps leading down into the ground. - After all, it looks like the entrance to some kind of base! - whispered Maks excitedly. Without further ado, they decided to go down. Kuba went first, Lena followed, and Maks closed the parade. They went down a few steps until the door appeared to their eyes - an old, iron door covered with drawings. Kuba reached for the handle, but then he heard a sound, as if someone was quietly whispering his name. For a moment, everyone froze. Then a soft blue glow appeared on Kuba's hand. Lena sensed an unknown force on her hand. And Maks felt that he was becoming very light, as if he could fly up in a moment. The friends looked at each other with horror and excitement. The door shook, as if someone was standing on the other side waiting for them to open it....