In Jecheon Duksan Elementary School, below Woraksan Mountain, 15 students have been in the same class for six years. They don’t sit still during the class like any others. After school, they would gather at their secret place by the river. They would whisper about their teacher that he can’t even control his own emotions. The new teacher, Yun-jae, who imagined his students to be calm and well-behaved, is frustrated to see them quite different from any other students. The children are now heading toward graduation. Their seasons fly in the sky. What is the sky filled with?