That day is the first afternoon of summer vacation and Yehu, Xiachan, Huya and Dimei all move ahead for the Monster Mountain. When they get to the top, they start to play monster catches kids. Xiachan first falls and she is the Monster! Accident happens when she catches others. Pals are frightened and all run away A year later, Xiachan writes a letter to her three friends, asking them to return the mountain and finish the game. Only in this way, she would turn into a human girl again.