This drama is the first Korean TV series filmed in the National Intelligence Service. Lee Soo-hyun (Lee Jun-ki) was adopted by NIS official Kang Jong-ho (Lee Ki-young) after his mother was killed by the Thai criminal organization "Green Gang". Together with his biological son Kang Min-ki (Jung Kyung-ho), he became an important member of the NIS. Soo-hyun always thinks of the nightmare in Thailand because of the war between him and the "Green Gang", which leads to the failure of the formal plan. NIS official Minister Zheng proposed a plan to infiltrate the "Green Gang" to Soo-hyun who was in chaos. Pretending to have died in an accident and concealing his past, Soo-hyun infiltrated the "Green Gang" for revenge. He used the fake identity of Kay to appear in the alleys and dark corners of Bangkok and became an undercover important member of the NIS. Soo-hyun's undercover identity also made him leave his lover Seo Ji-woo (Nam Sang-mi). She thought Soo-hyun was dead and accepted Min-ki's love. Just as she was trying to forget Soo-hyun, Kay, who looked exactly like Soo-hyun, appeared in front of her...