In the town of Nadwiśle everyone knew the Old Bridge - wide, wooden, creaking with every step. Julek, Kinga, Bartek and Zosia passed under it every day coming back from school. But one stormy afternoon, when the wind chased away the clouds and the puddles glistened like silver coins, they noticed something unusual. A rusty key and a piece of yellow map lay between the boards. Julek pulled out both objects, even though his hands were trembling. The map was drawn on an old napkin, and its lines led... right under the bridge! - Surely this is some kind of joke! - stated Bartek, although his eyes were glowing with excitement. - There is nothing there, just mud and old tins - said Zosia, but she agreed to check anyway. With the help of the torch from Kinga's phone, they illuminated the dark corners under the bridge. The key fitted into a small padlock by a metal door hidden behind stones. Silently, they all looked at each other. The lock clicked and the door swung open with a groan, revealing a darkness from which came the murmur of... Someone was on the other side. Or something. The friends froze in stillness, ready to take the next, most important step of their adventure.