In the year 2016, Lee Ha-Min (Seo Ji-Hoon) and Han So-Mang (So Ju-Yeon) attend Seoyeon High School. They keep their friendship as a secret from other people. Lee Ha-Min appears like a bright person, but there is also darkness within his mind. Han So-Mang is a warm-hearted person, but she is also quite timid. As they enjoy their high school days, something happens to them and this leads to Lee Ha-Min's death. In the present day and after six years, Han So-Mang returns to Seoyeon High School as a trainee teacher. There, she meets Lee Jae-Min (Kim Min-Kyu), who is the younger brother of Lee Ha-Min. Lee Jae-Min wants to know more about his older brother and his death. Other students around them includes Yoon Bo-Mi (Kang Hye-Won), who is a sweet model student, Choi Jin-Young (Yoon Hyun-Soo), who loves playing games and has a delicate personality, and Kang Sun-Hee (Oh Yu-Jin (1999)), who seems to have a strong personality, but she is actually tender-hearted.