During the Joseon Dynasty, the Secret Royal Inspectors are the eyes and ears of the king. They travel the provinces undercover and listen to the plight of the common people, investigating abuses and corruption of government officials.

To combat rising violent crimes, the Police Chief asks Detective Oh Goo Tak to form a team consisting of criminals. He is currently suspended from the police force for using excessive force. He then gathers team members: Park Woong Chul who is a gangster, Lee Jung Moon who is the youngest serial killer with extraordinary intelligence and Jung Tae Soo who is a contract killer. Police Inspector Yoo Mi Young also joins the team and she tries to have these guys work as a team by dealing with them rationally and sometimes emotionally.