For years, divorced and deprived of any intimate relations, unable to find a casual partner even now, Saya lives as a frustrated young divorced wife, secretly masturbating during daylight hours. While indulging in her usual self-pleasure, she is suddenly visited by Reina, her ex-husband's older sister. "I'm really sorry... such a disappointing younger brother," Reina says, approaching Saya as if they were family, then forcefully drawing her into her own lesbian world. Through her warm yet piercing words and actions, Reina gradually stirs Saya's emotions, guiding her toward a new awakening...