IESP-666 — I Fell In Love With My Husband's Sister. Lesbian Debut. Mai Kamisaki.

Mai has been in a long relationship with her senior at university and has just married him. Her husband has a younger sister, Hibiki, who is about the same age as Mai. One day, Hibiki shows up at the newlyweds' house. A few days later, Mai's husband goes on a long-term business trip overseas. Hibiki comforts the lonely Mai. Mai gradually becomes attracted to Hibiki.