Jun Igarashi attends Seikyo School as a high school student harboring secret romantic feelings for his teacher, Keiko Nagumo. He engineers opportunities to be near her by joining the school's badminton club, which she chairs. With no other members in the club, Jun and Keiko spend significant time together, their relationship deepening. His core motivation remains this concealed infatuation, deliberately positioning himself close to Keiko through club participation.

The situation intensifies when Keiko's daughter, Sae, approaches Jun. Sae poses probing questions about why her mother frequently mentions him at home, signaling a noticeable shift in Keiko's behavior. This encounter introduces tension regarding the secrecy of Jun and Keiko's evolving connection, as Sae's scrutiny threatens potential exposure. Jun's involvement now extends beyond pursuing Keiko to managing the challenge of discretion under family observation.

Jun's attraction is described as a "taste for forbidden, ripened fruit," highlighting the socially taboo student-teacher dynamic. Despite Sae developing her own romantic interest in Jun, his focus stays fixed on Keiko. This creates a triangular conflict, positioning Jun at its center and driving the unfolding drama between the three characters.