Franek had always enjoyed going to the woods, especially in summer when the sun was breaking through the leaves and the air smelled of resin and adventure. This time he was with his sister, Lena, who usually preferred books to expeditions, but today something touched her and she set off after her brother. Their favourite spot was an old oak tree on a hill - a tree so huge that even two children would not embrace it together. As they sat on the roots, Franek noticed something unusual between the mosses - a fragment of a metal doorknob. He pushed the moss away and Lena began to help him shake the ground with her hands. After a while, a door emerged. They were small, as if intended not for humans but for someone much smaller. They had carvings depicting strange creatures with feathers and scales, and a blue stone glittered in the middle. - Maybe it's better to leave them? - whispered Lena. - After all, we have to see what's inside! - replied Franek, already excited. He pushed open the door, which opened surprisingly easily, creaking quietly. Behind them stretched a tunnel, illuminated by a light reminiscent of moonlight. The tunnel was cool and there was a smell they had never smelt before - a little like vanilla and a little like smoke from a campfire. Lena, though a little frightened, held her brother's hand. With each step the tunnel widened, until suddenly the children stood on the threshold of a huge cave filled with amazing things: glowing crystals, flying skylights the size of the palm of your hand and a lake whose water sparkled with all the colours of the rainbow. Strange silhouettes floated above the lake - creatures with wings and tails they had never seen in any book. Just then they heard a quiet voice coming from somewhere above the surface of the lake: - What are you doing here, human children? The two friends froze, undecided whether to answer or go home. But before they could make a decision, something began to move on the other side of the lake....