My girlfriend Tsugumi and I started dating in college, and now we're living together with marriage in mind. She's kind, gentle, and has such a warm heart. I believe marrying her would be the greatest happiness in life, and I'm determined to protect that happiness with everything I have. But without my knowledge, Tsugumi has gotten involved with my best friend, Tsukasa. I always thought things like "NTR" or "cuckold" were just things I saw online—nothing that could ever happen to me. Yet suddenly, that reality hits me. It begins with a blowjob, slowly awakening my erection, stirring both frustration and arousal within me. How will my relationship with her change from here on?