Tập 1: Nao phát hiện mình là con nuôi và tìm kiếm sự thân mật với người anh trai cùng cha khác mẹ Shūsuke

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Nao Takanashi has long been trying to tempt her brother, Shūsuke, into entering an immoral (i.e. incest) relationship with her, with Shūsuke barely being able to resist. Nao goes to great lengths for Shūsuke to think of her perversely by doing things such as letting him purposely look underneath her skirt (and once letting him see her completely naked) but then she always gets him into trouble afterward so as to make it look like Shūsuke is the immoral pervert. One day, while Nao searches through Shūsuke's room for his collection of porn magazines, she then finds a hidden family photo album and is surprised to find out that there are no childhood pictures of her in it. After Shūsuke was finished being scolded about his porn collection, Nao confronts her parents, Shūji and Nanaka, about the album. They then tell her the truth: 10 years prior to the story, a car accident killed Nao's real parents and left Nao with memory loss, so Shūsuke insisted that Shūji and Nanaka adopt her into their family where he vowed to care & look after her as his sister. Nao, curiously enough, is more shocked by the fact she and Shūsuke are not actually blood related. At that instance, Nao realizes that a possible intimate relationship with him would be acceptable. Nao then requests Shūji and Nanaka to keep the fact she learned the truth from Shūsuke so that they can retain their sibling status. (Source: Wikipedia)