On a scorching summer day, 13-year-old Kouta Ochiai encounters a tall and alluring beauty. When their eyes meet, the mysterious girl takes Kouta's ice cream and calls him a pervert, which causes him to flee in embarrassment. Shortly afterward, he runs into his father, who is leaving for an urgent business trip, forcing Kouta to live alone for a while.

Overhearing the conversation, the girl Kouta met earlier reappears and wishes to compensate for the ice cream by looking after him. Although reluctant at first, Kouta eventually gives in to her persistence and allows her to stay in his house. The girl introduces herself as Natsuki Hagiwara, a 17-year-old high school student, but she remains silent about her motive and personal history.

Natsuki's presence immediately spices up Kouta's life, and as every day becomes more racy and exciting, he gains self-confidence with her by his side.