Behind a large meadow, where butterflies fly and crickets play, grew the oldest tree in the area. Underneath it stood an unusual school - the School Under the Big Oak. Tymek and his best friend Maja studied there. Each class was different: one had polka dotted walls, another smelled like strawberries and the third hid... on the colourful stairs! One morning, Timothy ran into school faster than usual. He couldn't wait for his lesson in stair class: he loved sliding down the yellow step and hiding behind the blue banister. Maja was already waiting for him, waving him down from above and calling out: "Come on, I saw something strange!". Tymek climbed the stairs and saw that one of the steps was moving to the rhythm of a song that the teacher, Mrs Basia, was humming quietly. That step had never moved before! Suddenly, a quiet tapping sounded from the floor. The children looked under the stairs and saw a shiny key lying on the red carpet. Where had it come from? Had it always been at school? Mrs Basia looked at the children and winked communicatively, as if she wanted to show them something even more important. A shadow appeared on the wall and began to dance and point to a small, closed door under the stairs.... Tymek and Maja fell silent. What happens when they open that door?