Because the president suffered a terrorist attack in Myanmar, the Ministry of National Defense decided to retaliate against North Korea. The action was led by Defense Minister Cui Yingcan, but 21 special forces members were killed after completing the mission. Under the protection of his friend Park Wulie, Li Zhenbiao (played by Kim Sang-joong) became the only survivor. Faced with national betrayal, he decided to take revenge. He snatched Park Wulie's only son and fled to Thailand to raise him. 28 years later, Li Runcheng (played by Lee Min-ho), who returned from MIT in the United States, returned to South Korea with his adoptive father's revenge. In order to conceal his identity, he was a special expert in the Blue House National Communications Network. By mistake, he met the Blue House guard Jin Nana (played by Park Min-young), who had a miserable life, but was quite strong. Similar fates made the two have a life-and-death love. Li Runcheng used the code name "City Hunter" for revenge and became the target that Seoul prosecutor Jin Yingzhu (played by Lee Jun-hyuk) had been pursuing. While eliminating the masterminds one by one, the truth is surfacing...