[From JanimeS]:

Se Young was a high school girl who has peaceful days until recently a worry begin to bother her. She had a childhood friend, apparently a boy, who also goes to the same school. His name is Heun Uoo. (Note : Their last names are not discussed here). Being a childhood friend, Heun Uoo does not have a feeling for Se Young, at least not as a female. However, Heun Uoo does care of Se Young in a way that Se Young is not satisfied of.