The Miyoshi family is a three-generation household. The grandfather, Senkichi, is incredibly virile for his age. His son, Sugio, on the other hand, is a serious man who is completely devoted to his work. Even when his wife, Reiko asks for sex, he only says he's tired. The couple has twin siblings, Sakura and Tsubaki, who are high school students. Sakura is like her serious father and does well in school. On the other hand, her younger brother, Tsubaki, has inherited his grandfather's traits and is more interested in women than studying. He brings home Natsumi, a female college student he met at a telephone dating service, and spends his days having sex. One day in the afternoon, Senkichi, looking for an outlet for his sexual desires, ends up having sex with Reiko, who is sexually frustrated...