Lee Soo Hyun and Kang Min Ki are childhood friends who grow up like brothers after Soo Hyun's mother is murdered by a Thai crime syndicate, the Cheongbang, and he's adopted by Min Ki's National Intelligence Services agent father. Years later, both become NIS agents, and Soo Hyun falls in love with Seo Ji Woo, who he first met when they were children. When Soo Hyun goes undercover to infiltrate the Cheongbang and avenge his mother, he suffers amnesia during a mission. Believed dead, he assumes the identity of a ruthless gangster named Kay. Min Ki, unaware of Soo Hyun's survival, tracks Kay, leading to a dangerous cat-and-mouse game where past loyalties clash with present realities, blurring the lines between friend and foe, and love and revenge – especially when hidden truths about Ji Woo’s past come to light.