Luhan and Seohyun were both childhood friends who grew up together in the same neighborhood. Luhan swores to Seohyun as they were kids one day that he'll never leave her side and that he'll always be there for her to protect her from any harm that comes her way. As the both of them start growing up, Seohyun realizes that she is developing feelings towards her childhood friend whom she took has her oppa since she was the only child in her family. Seohyun is scared to confess her feelings to Luhan because he sees her as a younger sister only. Will Seohyun be able to confess her feelings or keep it hidden forever. What happens when she finds on Luhan's birthday that he has a girlfriend already? How will things turn out for the both of them....