Kirino Kousaka embodies the ideal student with equally entrancing looks. Her grades are near perfect, and to cover her personal expenses, she works as a professional model alongside her best friend Ayase Aragaki, who abhors liars and all things otaku. But what Ayase doesn't know is that Kirino harbors a deep, entrenched secret that will soon be brought to light. At home one day, Kyousuke, Kirino's perfectly average brother, stumbles upon an erotic game that belongs to none other than his seemingly flawless little sister. With her reputation at stake, Kirino places a gag order on her sibling while simultaneously introducing him to the world of eroge and anime. Through Kirino, Kyousuke encounters the gothic lolita Ruri Gokou and the bespectacled otaku Saori Makishima, thus jump-starting an entirely new lifestyle. But as he becomes more and more involved in his little sister's secret life, it becomes that much harder to keep under wraps.

After receiving a flyer from Maina Ichii—a member of the idol group ChamJam—and seeing her perform at a local concert, Eripiyo falls in love with Maina and decides to devote her life to supporting the shy and reserved rising star. Eripiyo's charismatic presence and iconic red tracksuit soon give her recognition as Maina's one and only unrivaled fan. Whether it be a performance or a small fan gathering in the middle of nowhere, Eripiyo is guaranteed to be at every event Maina participates in. Not even an injury or Maina's seemingly cold attitude toward her can stop Eripiyo from pursuing her dream of seeing her beloved idol perform at the world-famous arena Budoukan.

The true end arc of Ore no Imouto. These four episodes branch out after the 11th episode of the main TV series and present an alternative version to the end of the TV series. These episodes contrast with the good end arc of the TV series, which was an original ending written for the anime, and instead closely follows the original story from the light novels.

The diehard otaku Kirino Kousaka has returned and settled back into life in Japan with her friends and family. Despite what her older brother Kyousuke has previously done for her, Kirino continues to give him the cold shoulder, much to his frustration. He is worried that his persuasion for Kirino to drop her track and field training in America and return home may have severely strained his relationship with her. On top of that, Kyousuke now also has to decode a bold and cryptic message from Ruri Kuroneko Gokou, his junior at school as well as Kirino's friend. As the ties between the two siblings and their friends deepen, Kirino and Kyousuke will soon have to figure out how they want to deal with these relationships, helping each other realize their own feelings in the process. In spite of that, Kirino still manages to find time to satisfy her otaku needs with the company of her brother.