Kangin and Sungmin were reincarnated but only Sungmin remembers the memories from their past lives and he was miserable because he had loved Kangin all this time but they were either friends or strangers in their past lives.  It also doesn't help when they're both born as males so Sungmin kept wishing that he would be reborn as a girl but will he even have a better chance being with Kangin as a girl?