It's a flashback about Sungmin's story with Sunny,his so-called-besfriend.Sunny had a crush with Sungmin but she never tell him about it.Everything going well until a new student,Jessica, appeared in Sungmin life.Sunny was more heartbreak when her bestfriend,Yuri, started to walk away from her life.

watch this:

http://www.youtube.com/watch?feature=player_embedded&v=vTOJLiGXItk