Set in the Joseon Dynasty during King Jeongjo’s reign, this martial-arts fusion sageuk is centered around the events involving Crown Prince Sado’s conspiracy. Warrior Baek Dong Soo was a real-life legend who created a martial arts guide in Joseon. His group who defends King Jeongjo is pitted against a mysterious organization of assassins who plot to kill the King.

Dong Joo and Da Ji were once a couple who were married at 19 but they quickly divorced. Years passed, they both return to their old house and discover that their feelings for each other have been endured.

The Homicide Investigation Team (H.I.T.) led by Cha Soo Kyung was established to investigate the case of a serial killer. The other four members were assigned from different precincts and when they first started, they all have their own ulterior motives and none of them was confortable with working as a team or trusted each other. Added to the mix, a rookie prosecutor Kim Jae Yoon was sent by the District Attorney's Office to keep an eye on the police's work. This drama depicts how cases were solved and how the members evolved into really working as a team.

Choi Kang (Eric Mun), a freeloader at home, has a hard time finding a job after being expelled from high school for fighting. He is admired by the gangs for his fighting ability, but his younger brother, working in the prestigious LK Corp., looks down on him. Then a little luck, and a bit of fortune lands him a meeting with the President and a job at the KSA (Korean Secret Agency)...and perhaps a girl or two are coming his way as well.