The story takes place in Seoul, 2011. Lee Yoon-sung is a talented MIT-graduate who's working on the international communications team at the Blue House. He encounters dangerous situations while solving a variety of cases, both big and small, for people who need his help, and eventually becomes a "city hunter."

After his mother is brutally murdered by the Triad in Thailand, Soo-hyun is adopted by NIS agent Kang Joong-ho, and he grows up with Min-ki like brothers. Min-ki and Soo-hyun both become NIS agents later on. One fateful day, Soo-hyun meets Ji-woo after last seeing her in Thailand when he was a child. They start dating. One day, Soo-hyun ruins a mission when the suppressed memories of the day his mother died surfaces in his mind. NIS Director Jung gives Soo-hyun an undercover assignment to infiltrate the Triad and Soo-hyun takes the job. Soo-hyun fakes his death and assumes a new identity to become a member of the Triad. With his fake identity and new name, he prowls the back alleys of Bangkok.