Ten years ago, Kogiku married my father and became my stepmother. My dad, who used to be cheerful and full of energy, passed away two years ago, leaving Kogiku looking rather lonely. Lately, though, she's started regaining her spirits. She's reopened the tutoring school she used to run from home and gradually returning to her old routine. However, the business has suddenly become extremely busy, and I've grown concerned about her health. I'd heard rumors that Kogiku had started some kind of special private lessons, which made me suspicious. When I secretly checked on one of these sessions, I discovered my stepmother caught in an unbelievable situation...