Eun-gyeol, a seemingly ordinary high school student, leads a clandestine double life. By day, he is a model student, but by night, he becomes a guitarist for a band in Hongdae. Unexpectedly, he travels back in time. Together, they form a band called 'Watermelon Sugar' and forge a deep bond. Along the way, Eun-gyeol encounters an enigmatic girl and falls in love. This tale intertwines elements of coming-of-age, fairytales, and magic, creating a captivating story of growth and self-discovery.