Kyosuke Kosaka, a normal 17-year-old high school student living in Chiba, has not gotten along with his younger sister Kirino in years. One day however, Kyosuke finds a DVD case of a magical girl anime which had fallen in his house's entrance way. To Kyosuke's surprise, he finds a hidden eroge inside the case and he soon learns that both the DVD and the game belong to Kirino. That night, Kirino brings Kyosuke to her room and reveals herself to be an otaku with an extensive collection of moe anime and younger sister-themed eroge she has been collecting in secret. Kyosuke quickly becomes Kirino's confidant for her secret hobby.