Kun is a boy born to an executive mother and an Architecture father. The family lives in a stepped house in Isogo-ku, Yokohama,Yokohama that Kun's father designed around a tree, where Kun spends his days playing with the family dog, Yukko, and his beloved toy train sets. When Kun is four, his sister Mirai, Japanese language for "future", is born, and he is happy at first when his mother returns home with her. But he soon grows jealous when his parents focus all their attention on her, and has to be restrained from hitting her with one of his toy trains. He lashes out first at his mother, and then at his father when he becomes a stay-at-home dad working from home while his mother returns to work.