Narcotic Squad leader HONG is suspected in a drug-related incident. Sung-joo, a young investigator, was commissioned by HONG to go undercover as a member of a gang called ‘Piercing’. Sung-joo leaves his cover and returns to the squad to help clear detective HONG of dishonor. On the 4th day of his investigation, he meets Yoo-jin, a traffic reporter who has the key to this incident..