Outside the town, right next to a huge old oak tree, stands the smallest school in the world. Lena, Wojtek and little Ignacy go there. One day, when lessons were coming to an end, Lena noticed that the clock in the school corridor was going.... backwards! Nobody had seen it but them. When they told Anna, their teacher, she only smiled mysteriously and said not to always believe the clocks. During the break, the children found small footprints in the courtyard that led straight to the big oak tree. From there there were barely audible sounds, as if someone was whispering very quietly. - Maybe it's a hedgehog? - Ignacy asked. - Or Mr Stasio's old cat? - guessed Wojtek. But Lena shook her head. - I've never heard such a whisper. Together they decided to come back here after dark, when the adults were gone. They came with their torches and... they saw that something flashed under the roots of the oak tree. They turned quickly - the sound repeated itself louder, and from underground emerged....