The Copernicus School had always seemed ordinary - until that evening, when three friends, Lena, Igor and Maks, stayed after lessons to finish their physics project. The teacher had long since left, the corridors were deserted and the echo of footsteps sounded ominous. Lena didn't like basements, but it was there, under the lab, that they had to find the box with the parts for the rocket model. As Maks illuminated the stairs with the torch on his phone, they noticed something strange - a thin line of light cutting across the floor behind an old mop wardrobe. Igor, curious as usual, pushed the cupboard away with a groan. The floor revealed an iron door, covered with strange symbols. Lena felt a shiver run down her spine. - What is this? - she whispered. Maks knelt down and looked at the symbols - one of them reminded him of a mathematical formula, the kind he had read about in science books, but there were also strange signs, as if.... out of this world. Igor, ignoring their warnings, pulled the metal handle. The door creaked and opened, revealing a spiral staircase leading downwards. A noise came from inside, as if the wind was swishing through unknown tunnels. At the end of the staircase, a blue glow shimmered. They descended, slowly, their hearts pounding like a hammer. As they stood at the foot of the staircase, they saw something unbelievable: in the middle of the dark room, on the concrete floor, a transparent surface swirled, as if a sheet of water that reflected another sky - purple, with three moons. - 'A portal,' whispered Maks, 'that it's possible...' - Lena looked at the others, seeing the same mixed feeling of fascination and fear in their eyes. - Are we going through? - asked Igor and took the first step towards the buzzing, pulsating membrane. At the same moment there was a strange sound somewhere behind them - as if something had moved in the shadows of the cellar. They froze, trying to decide whether to run away or enter the portal, not knowing what - or who - had just appeared downstairs with them....