A storm rumbled outside the window as Zosia browsed through dusty volumes in the ruins of the city library. Together with her friends - Eric, who always had a torch with him, and Amelia, who could fix anything, even a computer from the 1990s - they spent their holidays helping to renovate the old edifice. It was a building with a dark atmosphere, full of labyrinths, narrow staircases and nooks and crannies that few people remembered. This time, Erik found a small iron box with a strange engraved design in the basement. He held it in his hands, analysing the letter-like characters that they didn't even recognise on the internet. "Maybe it's just an old lock or a toolbox," Amelia suggested, but Zosia was convinced that something didn't add up here. When she tried to open the lid, she felt a slight tingling in her fingers. A moment later, a bright circular glow appeared on the wall just behind the box. The kids exchanged frightened glances. The light swirled, as if someone had thrown silver dust inside and made it dance to the rhythm of an unknown tune. "Look!" shouted Erik suddenly. They saw shadows on the floor that arranged themselves into familiar shapes - a table, a desk and even a glittering globe, as if they belonged to someone who used to work here. The glow on the wall widened until it finally formed a gateway. Beyond it shimmered a completely different world: strange trees with glassy leaves, a blue sun, a river flowing upwards. "I think it's a ... portal," whispered Amelia, looking on in disbelief. Zosia already knew what she was going to do. Before anyone had time to stop her, she carefully extended her hand towards the glow. Her hand disappeared, as if she had immersed it in the warm light. Her heart beat faster, and Erik and Amelia froze, not knowing what would happen next. Zosia looked over her shoulder at her friends. "I won't leave you," she said quietly, though she wasn't so sure herself. She took the first and then the second step until she disappeared all the way into the portal. Behind her, the light shimmered even stronger, and Erik and Amelia were faced with a choice - should they move after her, or go back upstairs for help? Suddenly there was a faint noise in the library, as if voices and unfamiliar sounds were coming from the depths of the portal....