8-year-old Moonbyul meets her brother's best friend Yongsun( 6 years older) and tells her she will be her wife someday, which seems cute in the teenager's eyes. Minhyuk and Yongsun go through many things together and date and break up several times without ending their friendship.  After living abroad for a long time, Yongsun gets back to Seoul and bumps into the Moon family, She is shocked when she sees 22-year-old Moonbyul and hears from her ex-boyfriend that the girl is the terror