Raspberry Forest has always been a place of fun and adventure for the children of Brodka village. This summer, however, something had changed. Lena, Maks and Filip discovered this one afternoon as they walked their bikes towards the clearing where they usually made base. The wind was rustling through the leaves just as it always did, but it had something new with it - a strange, cool gust that seemed to lead them deeper into the thicket. Lena, the most curious of the three, stopped her bike at the most spread-out oak tree. "Did you hear that?" she asked, looking at the boys with a glint in her eye. Philip shrugged his shoulders, but Maks had already pulled off his helmet and started looking around. "I think it's just the wind..." started Filip, but he didn't have time to finish because suddenly there was a quiet, deafening rumble from under their feet. They looked at each other in alarm as the ground shook slightly, as if a giant had just passed somewhere beneath them. Lena crouched down and pushed away the moss by the roots of the oak tree with her hand. "Look!" - she whispered. A small iron key appeared among the roots. It had curved teeth and was all covered in a shiny green patina. Next to the key someone had scraped a mark in the ground resembling two intertwined letters - L and F. Maks carefully picked up the key, feeling its cool metal under his fingers. Just then a stronger wind blew, and in its noise the children heard whispers - indistinct words, as if someone had spoken them very far away. "They mean something..." whispered Lena, trying to understand. Philip rubbed his eyes as, for a second, it seemed to him that a figure flashed from behind the nearest bush. Maks looked at his friends with a serious look on his face. "We need to find out what this key fits into. And what the letters mean," he said firmly. Suddenly, a sound came from deep in the forest - the crackle of breaking twigs and the rustling of leaves, so loud that Lena jumped back. Something, or someone, was approaching fast towards them....