August in the town of Olszyn was hot and smelled of dust, warmed grass and ripe apples from the garden orchards. Lena, Kacper and Maks wandered around the estate every day, looking for an activity to pull them out of their summer slumber. This time they stopped under a huge, spreading oak tree, reigning in a small park at the crossroads. Lena was curious like no one else. Her chestnut-coloured hair was always dishevelled and glints of new ideas shone in her eyes. Kacper, tall and somewhat thoughtful, liked to listen, and Maks - his younger brother - had a head full of crazy plans. More than once they got into a little trouble because of this, although it usually ended in a volley of laughter. That day, Lena noticed something strange under the roots of an oak tree - a greenish box, barely visible from under the moss. It was shaped like an old post office: covered in brass, with a small opening. Lena knelt down and looked at the sentence written on it: "For those who can listen". - Have you seen this before? - She asked, patting the box with her hand. - Nope - shook his head at Maks - but cool! Maybe it's some old cache? Kacper ran his fingers over the moss. - Did you know there used to be a postman's alley here? Grandpa used to say that they used to put boxes everywhere. Lena threw in a piece of paper she happened to have in her pocket. She just wrote on it: "Hello. We are here. And you?" Instead of expecting an answer, they decided to wait until the next day. The night brought a thunderstorm and the morning brought dawn mists. When they met again under the oak tree, Lena was trembling with excitement, unable to wait to see what they would find. Maks imagined a letter from some secret society, Kacper tried to remain calm, but deep down he felt the same tension. The box under the oak tree was locked with a small brass key, which the previous day had been unobtrusive. But now, when Lena touched the lid, they heard a quiet crack and the lid opened by itself. There was no letter inside. Instead, there was a delicate green leaf on which someone - perhaps with something sharp, perhaps a claw - had carved the words: "I invite you tonight at midnight. Bring a light. Just the three of you." - Is this some kind of joke...? - Maks looked at his siblings, but saw that Lena had already made up her mind. Kacper could still hear the quiet rustling of leaves - as if someone was waiting on the other side of the tree. The estate suddenly seemed less familiar, as if there was something more lurking beneath the shell of reality than they had ever thought. At midnight, with torches and hearts beating faster than usual, they were about to set off under the old oak tree. No one expected what was to meet them there....