Akane Mihara is betrothed to Shin'ya's brother through a family arrangement, placing her at the same school as Shin'ya. Their shared childhood includes a pivotal promise to reunite as adults. Shin'ya reignites this vow by actively pursuing her. He recruits his sister-in-law Sayumi to master romantic techniques aimed at winning Akane. Following Shin'ya's application of these lessons, the two initiate a physical relationship. This intimacy steadily erodes the emotional barrier between them. Their deepening connection disrupts surrounding relationships, signaling wider social and familial repercussions. The core narrative focuses on Shin'ya's mission to secure Akane as his own, utilizing their childhood pledge and Sayumi's guidance to overcome their initial separation.