The Worst of Evil is Set in Seoul in the 1990s, a former DJ starts selling a new powerful drug nicknamed "Gangnam Crystal" in city nightclubs after mastering a gangster organization. Since the police know little about the origin of such drugs, in order to crack down on this rampant drug trafficking organization, rural police officer Park Jun Mu is assigned to sneak into the organization, only to discover that his wife, Yoo Eui Jung , also a detective, has volunteered to participate in this dangerous mission and seems to have an unspeakable past with the heinous underground drug king. Park Jun Mu in this drug-related mission, not only does he wholeheartedly fight the drug cartel, but also works hard to protect his wife's safety at all times.