After sending off his wife, who left to stay with her parents for childbirth, Kenji finds himself overwhelmed by anxiety and loneliness in his first taste of living alone. With no motivation to tackle housework or cooking, he begins to give up—until one day, his wife's older sister, Reiko Kobayakawa, suddenly shows up offering to help with the chores. Exhausted from struggling with domestic duties, Kenji gladly welcomes her assistance. However, Reiko has a hidden agenda: an intense attraction toward Kenji. Through repeated bold acts—exposing her big tits and initiating intimate physical contact—she stirs Kenji's desires until he loses control, fully erect and unable to resist.