"What is the right thing, and what is the wrong thing?
"Undercover" is the story of Han Jae-Hyun, a man who has been hiding his identity for 27 years. Jae-Hyun's wife became Korea's first CIO(Corruption Investigation Office For High-ranking Officials) chief with the sole purpose of seeking "justice" and goes forward with her principles. Because of Choi Yeon-soo and the people of his past who started threatening Han Jae-Hyun to stop her, Jae-Hyun must protect his beloved wife Yeon-soo and children from them and from his own secrets.
Through Jae-Hyun, who constantly asks, "what is the right thing?" and in the process of his desperate struggle to protect his family, "Undercover" tries to tell us what values we must let go of and what we must hold onto in the Republic of Korea, in which values are constantly changing.