Do Kyung Soo is a genius detective and also a criminal profiler, nickname D.O. His father was a well known police commissioner and respected by his subordinates. His good-looking and intelligence made lots of people envy him. Yet his nonchalant attitudes towards others, made them fear him. For others, Kyung Soo’s life is perfect and flawless. Unknown to others, Kyung Soo had one dark secrets, one of which could never be revealed to the world.

Kai is a young police detective with a special talent, known for his straightforwardness and full dedication to his job, yet he had a warm heart and loved by his fellow officers and superiors. He started his job as a traffic officers, yet he dreamed to be a detective in Violent Crimes Department. Due to his hard work, Kai take the test to become a detective. He passed the test and transferred to the Violent Crimes Department.

Two men with such a different personalities are forced to work together on violent crimes cases. What would happened when both of them met each other? Would they be able to cooperate as a team in solving every cases they had?