30-year-old Jo In-yeong (Kim), a teacher at a private Seoul school, is shown besotted with 17-year-old pupil Lee Seok (Lee Tae-seong) during one of her night classes. (Their respective ages become an important plot point later on.) After giving him a lift home, she blurts out her feelings, and the two talk in her car.

Back home, her live-in b.f., Jeong-woo (Kim Yeong-jae), seems strangely untroubled by her fascination for the youth. Cut to a pigtailed teenage girl, also called Jo In-yeong (Jeong Yu-mi), whose b.f., Lee Su (also Lee), dies in a traffic accident. At the funeral, she meets his twin brother, Lee Seok (Lee again), and a romance develops, with the youngsters eventually making love. This strand is crosscut with the developing story of the teacher and the pupil that eventually brings out the hidden rebel in In-yeong's nature. At the pic's halfway point, as the teenage girl goes to visit her b.f. in Seoul, the script completely reverses what the audience has been led to believe. It's the start of a complex series of connections between all the characters that presupposes a cyclical element to falling in love -- and how the same choices and mistakes are made across generations.