National Security System (NSS) Top 707 agents Hyun-jun and Sa-woo are blood brothers who swore to trust each other and unchanging friendship for life. They meet agent Seung-hee there and both fall in love with her. But Hyun-jun and Seung-hee become close and Sa-woo fights to hide his feelings for Seung-hee. One day, they are immediately sent on a mission to Hungary. When they successfully finish off their job with immaculate teamwork and are celebrating it, a message arrives to Hyun-jun asking him to meet in secret. Baek-san, an executive of NSS who had been waiting for Hyun-jun orders a solo mission. Despite the tight protection of bodyguards, Hyun-jun succeeds his mission. But before escaping, Hyun-jun gets wounded by his targets' head security bodyguards, and later he is shockingly betrayed by his most trusted friend, Sa-woo. Hyun-jun tries to escape with Seung-hee's help, but he ends up losing Seung-hee. And his car also explodes; they are both misled into thinking that the other had died. Hyun-jun gets saved by an unidentifiable 'Voice' and learns the secret identity of a group called, 'Iris'. 1 year later, with the two Koreas about to be reunified, Hyun-jun makes his return. After spending a nightmarish life of being betrayed and losing everything, Hyun-jun returns to get revenge on those who threw him into the living hell. With the help of the President, Hyun-jun returns to his post in the NSS. Starting a fight against 'Iris', the secret society that tries to interfere with reunification, Hyun-jun now puts everything on the line for the battle of his life.