Almost all the inhabitants of the town of Mygłowo avoided the Emerald Forest. It was said that the trees there hummed in a strange way and that lights appeared among the ferns, which no one could explain. But for Lena, Karol and Ada, this place was the most interesting corner in the world. Each of them had their favourite reason for visiting the forest. Lena loved climbing old oak trees and collecting acorns, which she later turned into magical medallions. Charles dreamt of finding a rare blue butterfly he had seen in an atlas, and Ada just liked to feel the soft moss under her feet and make up stories about forest elves. One afternoon, as the sun was slowly setting, the three friends noticed something strange. Right in the middle of the clearing where they usually played trick-or-treating, there were scattered pebbles arranged in a large, shiny circle. Each stone shone in a different, intense colour - green, blue, purple and pearl. - This has never been here before! - whispered Lena, coming closer. Charles leaned over to look at the stones and noticed that a mist was gently escaping from one of them, as if the stone was breathing. Ada picked up the brightest of them. At the same moment, the air around them trembled and the forest suddenly fell silent. A soft, resonant laughter came from the distance, and right in front of the children grew a a small figure dressed in leaves and dewdrops. - 'Welcome, Guardians,' she said with a smile. - Your adventure is just beginning. The children looked at each other in disbelief, trying to understand if this was all really happening. As the figure pointed to a path hidden between old ferns, Lena felt her heart begin to beat faster than ever. What awaited them in the depths of the Emerald Forest?