The old primary school on Grabowa Street looked like many others - brick walls, chipped staircases, the noise of children at breaks. But one wing, which had been locked up for years, caused a thrill of excitement among the pupils. It was said to be where strange sounds were heard at night, and lights, though long disconnected, sometimes flickered in the empty halls. Zuzia, a thirteen-year-old with a feisty fringe and inquisitive eyes, had always enjoyed solving puzzles. Michael, a year older than her, was the rationalist type - he didn't believe in any supernatural stories, but for Zuzia he was ready to check even the most improbable tales. One cloudy day, just after the last bell rang, Zuzia approached Michal with a gleam in her eye. - I saw something in the window of that old wing, I tell you! - she whispered excitedly. - Like a ... A gleam, but golden, not so ordinary. Let's go there after lessons! They couldn't enter through the main entrance. So they went around the building, jumped over a gate with a scratchy "No Entry" sign and quietly crept up to the old windows. One of them, covered for years with a thick layer of dirt and cobwebs, looked different today. A soft glow could be seen in the middle of the glass, as if someone had lit a lamp from inside. - Is something glowing there? - Michael, although pretending to be calm, also felt a slight uneasiness. Zuzia carefully pushed aside a piece of rusty netting and looked inside. Inside was an old bench, and behind it on the wall - something that hadn't been there before. The shadow was arranged in the shape of a circle, and its edges pulsed with golden light. - See! - she hissed. - 'That looks like... a portal? Before they could do anything, the circle flared and a sound they didn't know came from within - between the hum of the wind and the rumble of a huge invisible mechanism. Susie opened her eyes wide. - We have to check it out! - she whispered. Michael looked at her hesitantly, but seeing her determination, he pushed back the glass and entered first. Barely had he set foot on the creaking classroom floor, the portal light flared rapidly. At the same moment there was the sound of footsteps behind them. Someone was approaching the door...