Set in the 1990s, 'The Worst of Evil' follows Kang Jun-mo, a low-ranking police officer who goes undercover to infiltrate a massive criminal organization based in Gangnam that is responsible for illegal drug trade between South Korea, China, and Japan. To gain the trust of Jung Gi-chul, the ruthless and charismatic leader of the Gangnam Union, Jun-mo must dive deeper into the underworld, risking everything he holds dear. Complicating matters immensely is Jun-mo's wife, Yoo Eui-jung, a brilliant elite narcotics detective who volunteers for the same dangerous case. Unbeknownst to Jun-mo, Eui-jung shares a complex past with Gi-chul, creating a high-stakes psychological triangle laden with suspicion, jealousy, and betrayal. As the boundaries between law enforcement and criminality blur, Jun-mo finds himself slowly consumed by the very darkness he swore to eradicate.