Takuya Enoki loses his mother, Yukako, when she dies in a car accident. Takuya now lives with his father, Harumi, and his baby brother, Minoru. Takuya’s baby brother is only two years old and requires much care. Although Takuya is only a fifth grader, he cooks, cleans, sews, scolds, and does everything that is normally a mother’s job. Between his housework and looking after his baby brother, Takuya begins to feel a helpless gap between his friends who have fun everyday after school. The Kimura family blame him for Minoru’s constant crying. His stress gradually builds up and bursts at a point. Because of a mistake Minoru made, Takuya reacts violently. He cannot help feeling upset over Minoru, who is stressing him and taking up all his time.
One day, when Takuya picks Minoru up from preschool, he starts to walk faster imagining what it would be like to leave his little brother behind him. But soon, Takuya realizes what he has done and runs back. He finds Minoru with his eyes full of tears and a brutal dog is blocking Minoru’s way. Takuya dashes in full speed and saves his little brother. After coming home and seeing Minoru sobbing, Takuya realizes that Minoru cries because he is just lonely and becomes ashamed of himself. From then on, Takuya learns to love his little brother as family and his long life story of being a mother and a brother at once begins.