Tachibana Yui is blackmailed into a secret, submissive relationship with her guidance counselor, Kuwabara-sensei, after he films her with her boyfriend, Kou-chan. To test her "devotion," Sensei forces her to wear a chastity belt throughout Kou-chan's baseball tournament, subjecting her to intense orgasm denial. Following the team's loss, Yui's desperation leads her to abandon her guilt and fully surrender to Sensei's control, admitting that her desire for him has eclipsed her love for her boyfriend.
As summer break draws to a close, Yui finds herself caught between her innocent relationship with Kou-chan and her intense, illicit affair with her teacher. Sensei offers her a provocative deal: she may continue their sexual relationship only if she remains Kou-chan's girlfriend. Fully succumbed to her desires, Yui accepts the arrangement, leading to a double life of deception that persists long after the summer ends.