Ala and Bartek loved their summer holidays at Grandpa Leon's house. His old house stood at the end of a narrow street, and behind it stretched a garden full of apple trees and spreading hydrangeas. However, it was not the garden, but rather the mysterious, always locked cellar that had long aroused their curiosity. One sunny morning, while Grandpa was taking a nap, Ala whispered to Bartek: - Are we going exploring? Bartek didn't need to hear it twice. They walked quietly up to the massive door, which usually creaked, but today acted particularly softly. Inside, the cellar was cool and the air smelled of dust and old wood. At first, all they could see were shelves full of pickle jars and their grandfather's old bike. However, in the corner, behind a canvas potato sack, they noticed something unusual - a large clock with lots of spinning hands and colourful buttons. - 'It must be something important,' whispered Ala. Bartek touched one of the levers and suddenly they heard a familiar voice. It was Professor Windmill - his grandfather's neighbour, an eccentric inventor! He had recorded himself on an old cassette tape, which now began to play: "If you can hear this, you have found the Clock of the Past. Be careful! It's not a toy. Set the right date and press the red button. But remember - not everything you learn will be easy to understand!" Ala looked at Bartek and Bartek looked at Ala. Without a word of agreement, Ala turned the pointer to the year 1420, and Bartek pressed the red button. The world whirled, there was a buzzing in their ears, and the cellar disappeared like a dream. They opened their eyes and... they were standing in the middle of a busy market square. People in strange clothes were bustling around them, and the air was filled with the smell of fresh bread and the loud calls of traders. Bartek grabbed his sister's hand when a boy with a basket of eggs ran right past them, chased by an angry trader. - Where are we? - asked Ala in a whisper. There was no time to answer. A tall man in a wide coat emerged from the crowd and looked at them intently. Something unusual shone in his eyes. Before they had time to take a step towards him, the man suddenly spoke quietly to them: - You are here for the first time. You must be careful. Look for the sign of the clock... Suddenly there was a loud sound and something they had not expected at all began to approach the centre of the market....