At a young age, Donghyun's father was assassinated right in front of him. Being deeply wounded, Donghyun promised to his father's grave that he will surely avenge him by finding the killer himself. He is an assassin but disguises as a bodyguard which is the biggest mistake he had ever made. 

How would he react if he will find out that the one who ordered to kill his father is the one who hired him to be the bodyguard of his son?

Is he going to fulfill his promise or not? What if he had fallen in love with his enemy's son?