The story of the investigative team Number 5 specially organized to actively combat crime through stake outs, wiretapping, communications monitoring, and deep-cover infiltration. A secret police unit that bends the limits of the law in order to fight crime, the new department comes in addition to the existing departments for criminal investigations, special investigations, criminal affairs, and white collar crime. The members are Cha Gun-Woo, Jang Moo-Won, Jang Min-Joo and Choi Tae-Pyeong. They will hunt down criminals and investigate crimes while undercover (hence the title).