After a car accident, Gi Ha Neul wakes up at age 29 with only the memories of his 19-year-old self, losing an entire decade of his life. The biggest shock comes when he learns that during those missing years, he was in a seven-year relationship with his lifelong rival, Yeo Sae Byeok. While photos, messages, and even his own instincts suggest they were deeply in love, Ha Neul remembers him only as the enemy who overshadowed him throughout childhood. As fragments of memory slowly return, he begins to suspect that their relationship ended for a reason and that Sae Byeok may be hiding the truth.