Ji Changwook is a man who many describes as mysterious and hard to read. And to anyone who has had an opportunity to converse with him  are unfortunately not alive to tell their experience. Changwook is a cold, heartless paid killer that puts his actions to use more than his words.  His side kick and partner in crime, Seolhyun, is the only one he trusts. She's an IT genius who's perfect at everything from hacking to tracking. He has never felt nor received love but that soon changes when he meets an interesting woman, Jung Somin.

She, on the other hand, is a detective and is very passionate about her work. She's kind and empathetic but she can kick your . She always sees the good in people despite her work heavily questioning her belief. She's always backed up by her senior partner, Kim Seonho. He's her childhood friend and fellow detective. He may come across as clumsy and goofy but he's the best in the business. 

So what will happen when people from two different worlds come face to face with each other? Chaos or harmony?