Kim Hyoyeon is a one of the top agents of the SME. Being an agent, she's been through many things especially losing those she loves. Because of her job, she attracts many enemies, which results her to isolate herself from her 'non-agent' friends. So basically, her only family is her fellow agents. But just because she grew up with this type of life, she has a loving and fun personality.

Lee Hyukjae, also known as Eunhyuk is the son of an intelligent scientist. His father, Lee Dongwook was held hostage by the JYP agents. For years he was tortured and forced to make a hazardous toxic substance that can put the JYP agents in power and damage the world. When Lee Dongwook returned home, he made Eunhyuk promise something that can put his life in danger. Being the loving son he is, he did everything for his father until he was murdered. 

Because Kim Hyoyeon is the one of the best agents and Eunhyuk's life is in danger, they must stick together until the problem is resolved - meaning they both need to live in the same roof and be under SME's protection, Hyoyeon thought this babysitting mission was easy and simple, but she was wrong. Not only is she fighting for their life, but she's also fighting for her feelings. For Eunhyuk, ever since his father died, he's been distant and having trust issues. With Hyoyoen by his side, he learns to love and protect again.