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In Seoul, Korea, two sisters must look after each other when their mother leaves them to search for their estranged father.

Two elite National Security System (NSS) agents, Hyeong-Jun (Lee Byung-Hun) and Sa-Woo (Jeong Jun-Ho), who were sworn blood brothers, both fall for fellow agent Seung-Hee (Kim Tae-Hee). Sa-Woo attempts to suppress his feelings for Seung-Hee and wishes the couple the best. Hyeon-Jun is then sent off on a solo mission to Hungary. In Hungary, Hyeon-jun accomplishes his mission and while in escape mode becomes wounded. Shortly later, Hyeon-jun finds out that he was betrayed by fellow agent Sa-woo. Seung-Hee attempts to help Hyeon-Jun escape, but a car explosion separates the two and both are misled to believe that the other person are dead. Hyeon-Jun is then saved by an unknown voice and learns of the secret society "IRIS". One year later, Hyeon-Jun returns to Korea to seek revenge. During this time, the two Koreas are set to reunify, while the group "IRIS" are intent on stopping the reunification.