The series ended on June 12, 2021 with viewer ratings of 5.2% nationwide and 6.1% in the metropolitan Seoul area. It is its own best ratings achieved during its run.[10]
Han Jeong-hyeon (Ji Jin-hee) is an agent of the National Intelligence Service (formerly the Agency for National Security Planning), who has been hiding his identity for the past 30 years as a father and owner of a bicycle repairing shop. Choi Yeon-soo (Kim Hyun-joo), his wife and a Human Rights lawyer, becomes the first Director of the Corruption Investigation Office (CIO). These characters are trying to uncover the truth, pitted against a huge force holding a top position. The series tests their relationship as husband and wife when the secret life of Jeong-hyeon comes to light.
Lee Seok-kyu is agent at the National Intelligence Service, formerly the Agency for National Security Planning. He was selected as the new agent of NIS when he was in a police academy. At first he was tasked to spy on Choi Yeon-soo under the pseudonym Han Jung-hyun in order to take down Kim Tae-yeol who was targeted by NIS, eventually both of them were in mutual love and got married. Years later, he lives a peaceful life with his family until one day an NIS agent named Do Yeong-geol comes back to his life and he is enlisted to take down his own wife, Choi Yeon-soo.
A human right lawyer for more than 20 years. She passed her bar test at the age of 20. A righteous person, she consistently seeks justice for years to Hwang Jung-ho for 20 years after he was unjustly jailed. She joined a mass protest with Kim Tae-yeol, Kang Choong-mo and others to protest against the corrupted South Korean government who failed in protecting Hwang Jung-ho's innocence. In one of a chaotic protest against the government, she met Lee Suk-kyu, under the pseudonym Han Jung-hyun. Both of them ended in mutual love, without her knowing who Suk-kyu was, therefore getting married with him. Years later, because of her consistency in defending Hwang Jung-ho, she is promoted as the Corruption Investigation Office for High-Ranking Officials by the government. However due to this promotion, her husband is tasked to stop her.
Suk-kyu/Jeong-hyeon and Yeon-soo's autistic older son. He has an obsession with bicycles, therefore he helps his father operate their bicycle repair shop.
Suk-kyu/Jeong-hyeon and Yeon-soo younger daughter. A middle school student with maturity and intelligence beyond her age. She resembles her mother in everything
Lee Suk-kyu's father and a war veteran, being proud of his son who serves the country like he did in the past. He believes that Suk-kyu went to the US when actually he has to live under name of Han Jeong-hyeon. His health deteriorates as he waits for his son to come back to him. Since Suk-kyu cannot reveal his identity, he spends time with his father in the hospital, posing as a social worker. Man-ho never realizes that his son is always with him even until his death.
30 years ago, he was wrongly accused of a murder case and was sentenced to death. Therefore, while he was incarcerated on death row, Choi Yeon-soo is determined to seek justice for him as his lawyer. However his case hasn't got any shed of light for 30 years, even after his health severely deteriorated.
Kim Tae-yeol is a president of Hankuk University Student Council and leader of the mass protest against the government in 1991. Eventually he was accused as a communist and being targeted, then murdered by NIS.
Jang Eui-don as police university instructor (ep. 1)
Lim Jung-ok as Nurse Kim (ep. 1-2)
Kim Dong-ho as Cheon Woo-jin, Im Hyeong-rak's subordinate (ep. 2 & 8)
Jung In-gi as Kim Myung-jae, Secretary General of the Blue House
A man who claims himself being intentionally beaten by Han Seung-goo so that Yeon-soo would be hindered to be the new chief of Corruption Investigation Office for High-Ranking Officials. Actually at that time, he abused his girlfriend and being caught by Seung-goo who said that a bigger person might not hit smaller person.
Park Joo-hyung as drug gang boss (ep. 4)
Kim Young-woong as subway control room staff (ep. 3)
Lee Se-na as Kim Da-kyung (ep. 4)
Ex-girlfriend of Lee Min-yool. Being guilty with Seung-goo's situation, she confessed about what happened between her, Min-yool and Seung-goo to Yeon-soo.
In the table above, the blue numbers represent the lowest ratings and the red numbers represent the highest ratings.
This drama airs on a cable channel/pay TV which normally has a relatively smaller audience compared to free-to-air TV/public broadcasters (KBS, SBS, MBC and EBS).
NR denotes that the series did not rank in the top 20 daily programs on that date.
^"지진희X김현주, '언더커버'로 재회…내년 상반기 방송" [Ji-hee Ji x Kim Hyun-joo reunited with 'Undercover'... Broadcast in the first half of next year]. JoongAng Ilbo (in Korean). July 8, 2020. Retrieved November 27, 2020.
^"'언더커버' 연우진·한선화 합류…퍼펙트 라인업 공개" ['Undercover' Woojin Yeon and Sunhwa Han joined... Revealed the perfect lineup]. JTBC. July 30, 2020. Retrieved November 27, 2020.
^Lee Min-ji (March 17, 2021). "'언더커버' 지진희X김현주 포스터, 엇갈린 시선 너머의 혼란" ['Undercover' Ji-hee Ji-hee x Kim Hyun-ju poster, confusion beyond mixed gazes]. Newsen (in Korean). Naver. Retrieved March 17, 2021.
^"언더커버" [Undercover]. JTBC Studios. Retrieved February 14, 2021.
^Kim Ji-eun (April 16, 2021). "티빙, '언더커버' 독점 공개... JTBC와 협업 본격화" [TVING to unveil 'Undercover' exclusively... Collaboration with JTBC in earnest]. Newsis (in Korean). Naver. Retrieved April 18, 2021.
^Kang, Jin-ah (July 23, 2020). "'언더커버', 지진희·김현주·허준호·한고은 라인업 완성" [Undercover' completed lineup for Ji-hee Ji, Hyun-joo Kim, Jun-ho Heo, and Go-eun Han]. Newsis (in Korean). Retrieved November 27, 2020.
^Kim, Myung-mi (July 9, 2020). "연우진-한선화 '언더커버' 출연, 지진희-김현주 과거 연기(공식입장)" [Yeon Woo-jin and Han Sun-hwa appeared in'Under Cover', Ji Ji-hee-Kim Hyun-ju, past acting]. Newsen (in Korean). Retrieved November 27, 2020.
^Jang, Yoon-jung (July 8, 2020). "지진희-김현주, JTBC '언더커버'로 출연 확정 내년 상반기 방영" [Ji-hee Ji-Kim Hyun-joo confirmed to appear as JTBC'Undercover' "Aired in the first half of next year"]. Aju News (in Korean).
^Yoon, Sun Cho (July 30, 2020). "[공식] 유선호, JTBC 새 드라마 '언더커버' 출연 확정…지진희·김현주와 호흡" [[Official] Yu Sun-ho, JTBC's new drama'Undercover' confirmed to appear... Ji-hee Ji and Hyun-joo Kim]. Chosun (in Korean).
^hoon Hong, Sung (November 26, 2020). "차순배, 드라마 '언더커버' 캐스팅 결정" [=Cha Sun-bae, cast of drama'Undercover']. NBN News.
^Choi, Ji-eun (November 4, 2020). "임형국,'언더커버' 출연 확정...네이티브 못지않은 북한말 구사" [Lim Hyung-guk confirmed to appear in'Undercover'... speaks North Korean as well as native]. Top Daily.
^"안세호, JTBC '언더커버' 합류..정만식 조력자 활약 [공식]" [Se-ho Ahn joins JTBC 'Undercover'.. Jung Man-sik as an assistant [Official]]. Osen (in Korean). Naver. March 10, 2021. Retrieved March 10, 2021.
^Ha-eun, Jeong (April 26, 2020). "'부부의 세계'→'언더커버'·'엉클', 英 원작이 뜬다[SS드라마]" ['Couple's World' →'Undercover' and'Uncle', the British original is coming [SS Drama]]. Sports Seoul.
^Lee, Ji-eun (July 30, 2020). "'언더커버' 연우진·한선화·박경리 출연 확정…라인업 완성" ['Under Cover' Yeon Woo-jin, Han Sun-hwa, and Park Kyung-ri appearing confirmed... Lineup completed]. NewsPim (in Korean). Retrieved November 27, 2020.