Kagetsu Hitomi was living a happy married life, but her only concern was her husband's flirtatious younger brother. She felt uneasy about his frequent visits, especially during the quiet afternoon hours when her husband was away. That day, the brother-in-law offered a massage as an apology. Letting her guard down, Hitomi entrusted her body to him—only to be violated by her own brother-in-law. After that incident, he began using her as his sex partner, flooding her fertile womb with his young sperm every day while teaching her the pleasures of forbidden desire. The popular doujin work collaborates with the original series—introducing "Onee-san, Be My Fuck Buddy, OK?"