As the story of the brothers unfolds, the younger son, Takashima Yasuhiro, and his mother, Takashima Midori, stay at the home of her real sister and their aunt, Kuroyanagi Misako, while their house undergoes renovation. They live by renting the room of their cousin, Kuroyanagi Tatsuo, who attends university in central Tokyo and lives alone. Midori, treating the arrangement as a temporary residence, diligently handles housework and contributes to the household. Watching her like this, Yasuhiro gradually begins to realize his growing feelings. Meanwhile, the eldest son, Takashima Yohei, a busy elite salaryman, also starts noticing his mother more and more. One day, he becomes acutely aware of her stunning big ass. This shocking realization brings a dramatic shift to the family's dynamics.