After his mother gets brutally killed by the Thai criminal syndicate, Cheongbang, Suhyeon (Lee Junki) gets adopted into NIS agent Jungho’s family. Together with Mingi (Jung Kyungho), Jungho’s son, Suhyeon becomes an NIS agent. Suhyeon had met Jiwu (Nam Sangmi) when they were younger and later has another fateful encounter with her. Together they make promises for the future. However, one day, Suhyeon leaves for an operation dealing with the Cheongbang, only to return in failure because of the resurfacing of terrible memories when he was in Thailand previously. The chief of NIS agents, Jeong, proposes to the distraught Suhyeon to infilitrate the Cheongbang. So, through Chief Jeong’s proposition, Suhyeon becomes an undercover NIS agent. Thoroughly hiding his past, Suhyeon infiltrates the Cheongbang with revenge on his mind. Under the pseudo-name of Kay, he passes through the backstreets of Bangkok…