Julka had always thought of her school as very ordinary. Even its building - grey, with big windows and creaky stairs - seemed like all the others. But everything changed one dreary Monday when she received a new maths notebook. It was thick, bound in a light blue cover, and someone had scrawled an illegible symbol in chalk on the inside. Before Julka had time to sit down at her desk, Witek, her best friend, rushed into the classroom. He was all dishevelled and excited. He was looking around nervously, as if looking for something or... someone. - Julka, you have to see this! - he whispered, pulling her towards the blackboard. On its surface, between the equations written down by Mrs Ziętka, there were strange, glittering signs that pulsed with a faint light. Julka blinked to get a better look. Instead of the usual numbers and letters, the marks began to form maze-like patterns, and then - to their amazement - they slowly flowed across the blackboard, moving as if they had a will of their own. Suddenly, one of the equations broke away from the blackboard, twisted in the air and fell silently to the floor, turning into a tiny chalk ball. Witek bent down and picked up the chalk ball. At the same moment there was a quiet noise in the classroom, as if the wind had run between the desks. The ball unrolled in his hand, revealing a tiny, rippling mist inside. Julka watched in horror and fascination as the mist began to swirl faster and faster, until suddenly.... - Stop! - shouted Julka, but it was too late. The whole class swirled, colours began to blend and sounds became strangely stretched. In an instant, Julka felt as if she was in a completely different dimension. A door appeared in front of her and Witek - tall, bright and... locked with a key that wasn't there. Just where is everyone else? What is behind that door? Julka looked at Witek and knew that something extraordinary was about to begin....