She had came back to threaten her and her boyfriend. Threaten the two love birds that had been dating for two years and now trying to break that bond? Why? She happens to be Baekhyun's ex girlfriend, a terrible ex girlfriend to be exact.  

Baekhyun and Sulli, the two who got torn apart by his ex girlfriend. Sulli who had to leave his side to protect him. Baekhyun who had been depressed ever since then but tried to forget her but the face that he would always see every morning, would pop back into his mind. 

She is the only one that belongs into his heart and he, too belongs to hers.