Ola, Kuba and Lena loved spending time in their tree house, hidden in the thick branches of the old oak tree behind Ola's house. The cottage was a little crooked, smelled of resin and always seemed like a place where anything could happen. That morning the sky was streaked with heavy clouds and the air smelled of rain. - Today we shouldn't go there, it's about to pour! - protested Lena, clutching her favourite book to her chest. However, Ola was already running down the stairs and Kuba, clamping his mouth shut, followed her. When the drops began to tap on the roof of the cottage, they ducked inside. It became cramped and dark inside, and every thump made Lena jump. Ola laughed, trying to relieve the tension: - Imagine being on a ship during a storm! No one spoke up. Kuba sat in the corner, playing with a piece of old string. Lena stared out the window, her eyes glittering restlessly. Then, completely unexpectedly, a dusty old notebook fell off the shelf. It hit the floor and opened in the middle. Everyone looked at each other, then Ola was the first to reach out to pick it up. - 'I think this is my dad's notebook from when he was a kid,' she said quietly. There were drawings and short sentences on the pages, full of erasures and additions. Suddenly Kuba noticed a strange, hurried annotation at the very end: "Where the storm gives strength, hidden is something that changes hearts.... Find it before the light goes out." The children froze. Thunder rolled across the sky and the roof of the cottage trembled. Each of them felt their hearts beat faster - out of fear, excitement, or perhaps out of simple curiosity. What could this mean? How to find what Ola's dad had written about? And was the storm actually hiding something? Suddenly the wind closed the shutters with a clatter, and the children held their breath....