Natsumi harbors deep admiration for her best friend's older brother, whom she currently tutors privately. The brother constantly indulges in lewd fantasies, and as Natsumi grows increasingly attracted to him, she struggles to suppress her own desires. One day, while studying with him upstairs, she overhears her best friend having sex with her boyfriend in the living room. Stimulated by their sounds, Natsumi becomes overwhelmed with excitement and finally gives in, seducing him into having sex. This story depicts the gradual transformation of their pure tutor-student relationship into something more intimate. Entangled emotions between siblings and the complications of her role as a teacher deepen her feelings even further. A tale filled with hidden desires and secret relationships simmering beneath everyday life.