8-year-old Moonbyul meets her brother's best friend Yongsun( 6 years older) and tells her she will be her wife some day, which seems cute at 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 bump into the Moon family, getting shocked when she sees 22-year-old Moonbyul and hear from her ex-boyfriend that the girl is the terror among the ladies.

Moonbyul flirts with the 28-year-old and reinforces her childhood promise, causing strong palpitations in the other woman.But also some type of rejection.

Yongsun sees her friend's sister as a child and she has no intention to get involved with someone way younger and immature.