Just before the Obon holiday, during a punishing heat wave, Rumi travels to Tokyo because she is worried about her adult stepson living alone. She became his new mother while he was in school, and despite his coldness she patiently tried to reach him. Now old enough to live independently, he finally opens up, making her genuinely happy—but he has long desired her as a woman. That night, seeing her flushed and sweaty after a bath, he can no longer suppress himself, beginning two nights and three days of steamy sex and creampies in this adult stepmother-and-stepson role-play.