With their parents away on a business trip, the responsibility of watching over his younger sisters falls to their older brother, Souichirou Nanase. Summer vacation has just begun, meaning the siblings are set to spend considerable time together. However, Souichirou quickly discovers that caring for five teenage girls—Michika, Koharu, Hiyori, Rio, and Aya—could be more troublesome than he anticipated. In addition to their distinct and quirky personalities, the girls are at an age where young women often experiment with their bodies and ask "those" questions. To complicate matters further, Souichirou suffers from a sister complex, and the five temptations roaming around might be sufficient to finally shatter his moral barriers.