While their parents are away on a business trip, the one entrusted with watching over his younger sisters is the big brother, Souichirou Nanase. Summer vacation has just started, and the siblings are bound to spend a lot of time together. However, Souichirou soon realizes that taking care of five adolescent girls—Michika, Koharu, Hiyori, Rio, and Aya—may be more troublesome than he expected. On top of having distinctive and quirky personalities, the girls are at that age when young women tend to experiment with their bodies and ask "those" questions. To make matters worse, Souichirou has a sister complex, and the five temptations walking around might be enough to finally break his moral barriers.