The village of Nalecz lay on the edge of a vast forest, about which the elders told incredible stories. Lila, an energetic twelve-year-old, had never believed in tales of mermaids or forest spirits. However, on this summer afternoon, something drew her to the thicket. Together with her three friends - Filip, Zośka and her younger brother Miłosz - she decided to discover what was hidden behind an old oak tree, which according to their grandmother was a passage to "another world". They took torches, notebooks and raspberry jam sandwiches. Returning from a trip to the river, Philip noticed footprints between the bushes, as if someone had dragged a huge sack there. The footprints led to where five oak trees grew in a circle. In the middle of them the ground was black, as if someone had recently lit a bonfire there. Suddenly, Milosz pulled an old copper key out of the grass, and as soon as he touched it, all the trees rustled as if they were speaking an unknown language. - 'It's definitely the wind,' Zośka tried to joke, but Lila felt goosebumps. It became quiet around them, and glittering green blinds emerged from the dense bushes. From behind the largest oak tree came a soft, melodious chant. The children froze, watching as the mist began to swirl, arranging itself into a familiar shape.... And then...