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.

Nao coerces Shūsuke into going to the beach with her, though coincidentally they end up running into Haruka, Hirono and Iroha, who Nao recognizes as the pigtailed girl from Shūsuke's childhood photos. Nao and Iroha end up competing with each other for Shūsuke's attention, though it tends to just bring him pain. Nao accidentally knocks him unconscious with a rented boat, and he dreams of his favorite eroge character. Nao and Iroha compete with each other over who gets to perform mouth to mouth resuscitation, though in the end, he is revived by one of his male friends.

While running back from buying a new porno magazine, Shūsuke bumps into another person. They get their packages mixed up and Shūsuke ends up with a Boy's Love magazine that he immediately rips to shreds. When the other party, known as Mister X, discovers about the mix up, he sends Shūsuke a threatening note demanding its return. That night, when Shūsuke attempts to make an exchange with Mister X using a different BL magazine, with Nao and Iroha stalking him, he learns Mister X's true identity is that of his class rep, Mayuka Kondō. With the threat of his porn being shredded or shown to Nao, Shūsuke agrees to become Mayuka's pet, assuming there will be sexual requests involved. The requests start simple, such as buying her a drink or trying her bentō lunch. However, the next evening, Mayuka asks Shūsuke to buy her a more extreme BL magazine she's too embarrassed to buy herself.

After having Iroha spy on Shūsuke and Mayuka the previous night, Nao tries putting on black pantyhose and clinging onto Shūsuke as he sleeps. Iroha and Mayuka later arrive, leading to an awkward confrontation. Mayuka ends up questioning what Nao and Iroha see in a pervert like Shūsuke, mentioning her affection towards him is different from theirs. Later that night, Mayuka gets Shūsuke to read through one of her BL books, which ends up traumatizing him.