University student Minoru Niijima is a timid young man who gives up on his girlfriend's older brother after being threatened by him. Minoru's father, Seiichi, is a tired office worker nearing retirement, and his mother, Reiko, is an ordinary housewife who is considerate of her husband and believes her children are well-behaved. Minoru's older brother, Shigeru, works as a bed salesman. He is a nuisance to Minoru, as he is rough and always brags about his work. Minoru's younger sister, Yoko, is a second-year high school student, but to Minoru, she still has the childishness of a middle school student. One day, Minoru is persuaded by his university friend, Karaki, to commit a sexual assault on a train. Unfortunately, the girl he chooses is a female student from a delinquent group, and he ends up being beaten up by the delinquent group. However, the new member of the group who appears before Minoru is his younger sister Yoko.