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

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.

When Shūsuke gets a report about him failing in all of his subjects, he then realizes that he has to take a follow-up exam or risk having to take summer classes. Believing that she was the cause of Shūsuke's grade for calling him out each night, Mayuka feels guilty and offers to help him study for an upcoming exam. But while they were studying, Nao and Iroha decide to intrude on them. As Mayuka tries to get Shūsuke to study, Nao and Iroha compete to try and catch his attention. After some time later, Iroha presents surprise tickets for everyone to go to a local pool park if he passed, which encourages Shūsuke enough to pass. At the pool, Shūsuke spends most of his time teaching Mayuka how to swim, leaving Nao and Iroha pretty jealous. During that time, Mayuka becomes amorously focused with Shusuke's nipples, having never seen a man naked before.

Late on night, Shūsuke spots a sexy cosplayer named Ran Yatagai on top of a vending machine. He later sees Ran, who calls herself Princess Leila, at school the next day, who invites him and his friends to the rooftop. She claims to be seeking a saviour amongst the scum of the Earth in order to defend it, with Shūsuke and friends believe to be initiation for some kind of underground club. As Nao and Iroha decide to follow her, they meet her twin sister, Rin, who reveals Ran is using her own peculiar method to find a partner. While Rin manages to scare off the other AGE explorers, Shūsuke goes alone to an 'awakening ceremony'. Shūsuke is scared off by Nao's presence and runs off with Ran, who decides he is her saviour. Meanwhile, Rin appears and shows romantic interest in Nao, now that Ran is preoccupied.

As Shūsuke puts up with more of Ran's dubious rituals, Rin forces Nao to hang out with her by threatening to tell Shūsuke that she had been stalking him, looking for an opportunity to kiss her. Shūsuke and Nao soon become paranoid around each other when they see each other hanging around the respective Yatagai sister. Nao and Iroha determine that the only way to get Rin off her back is to convince Ran that Shūsuke isn't her saviour by pretending to be prophets on their mobile phones. Rin catches onto their plan and tries to get the drop on Nao but ends up kissing Iroha instead, though she immediately takes a liking to her lips and runs off with her. Ran later explains to Nao that she wanted to mentally train Shūsuke to play as a warrior for a cosplay event, though when the day of the event arrives, Shūsuke gets the address wrong and arrested for wearing his skimpy outfit in public.