Everyone have their past. Everyone have their secrets. But will the secrets be the reason behind your pain? Or will your secrets be your key to learn to love yourself? 

Seohyun have her very own secrets. She was known for her skills as she was the best killer in the agencies. People will scared from hearing her killer's name, Butterfly. But it all were in the past. Now, she was the best agent in the National Inteligience Agency and also a student at Seoul National University. Because of her past, she always in danger as her past enemy can always learn the truth about her. She never worried about her past until a boy makes her heart flutter for the first time in her life.

Namjoon also have his secrets. He is the shadow agent in the National Inteligience Agency and no one ever see his face. He the only one who can keep score as Seohyun and he is her partner. Instead be a field agent, he prefer being a hacker as he excel in it more than anyone else (except Seohyun). And now, he got a new mission as his partner is in danger because of her cover in previous undercover was unfolded. He need to protect her as she doesn't know what's await her. He went to Seoul National University as a student together with Seohyun and try to keep her closed. 

Together both of them learn the beauty of accepting someones good and bad and learn to embrace it.