The story of a certain family begins when the older brother, Tatsuo, goes on a long-term overseas business trip. During his absence, the younger brother, Yasuhiro—though he was supposed to have left as a son-in-law—ends up starting a relationship with his sister-in-law, Murakami Ryouko, shortly after moving in. As a result, Yasuhiro becomes Ryouko's sex slave. Meanwhile, the younger brother's wife, Leon Otowa, noticing that her husband no longer satisfies her at night, begins acting provocatively and seductively toward her brother-in-law, Tatsuo, eventually leading to an affair. Even after Tatsuo returns from abroad, Ryouko continues to seduce him, and the two brothers hatch a plan. Simultaneously, they make a strange proposal: "Want to test which is more of a pervert—my wife or your wife?"