Because I Don't Like My Big Brother At-All--!!

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.

It turns out that Nao and Shuusuke are not related by blood. While most people would be upset after finding out something like that, Nao took it in the most positive way possible - it meant that there was nothing to get in the way between her and Shuusuke. The next day, a new student named Iroha transfers into Nao's class. At first Nao gets along with her really well, and they even end up having lunch together... but while they are eating Shuusuke comes to give Nao her keys which she left at home. Even though Iroha and Shuusuke should be meeting each other for the first time, Iroha seems strangely interested in him...

Iroha coerces Shūsuke into going on a date with her, and Nao decides to follow them, while her friends, Haruka and Hirono, decide to tail her as well. Iroha takes Shūsuke various places including a movie theater, an arcade and a swimsuit store. At the end of the date, Iroha tries to get Shūsuke to kiss her, but he refuses. Iroha tries to drag him somewhere else, but Nao manages to bail him out. Nao begins treating Shūsuke coldly, leading him to realize that she knew about his date with Iroha. When Iroha scares Shūsuke with the notion that she knows everything he's been doing (having been watching him through a telescope), Shūsuke rolls on the floor when Nao enters, getting a look at her panties which reawakens his normal perversion, much to Nao's delight.