Best friends who are like brothers Kim Hyun-jun (Lee Byung-hun) and Jin Sa-woo (Jung Joon-ho), were training under the South Korean army’s 707th Special Mission Battalion until they were scouted by NSS (National Security Service). The duo was independently scouted by the beautiful NSS agent, Choi Seung-hee (Kim Tae-hee). Unaware they have been scouted, the two were secretly captured and subjected through a simulated torture test, and both passed their test to be worthy NSS agents. In the process, they also fell in love with Seung-hee, who has kept the scouting process a secret from them.
After completing the test, Hyun-jun and Sa-woo were introduced to the director of the NSS, Baek San (Kim Yeong-cheol). Baek San explained to them that the NSS is a secret organization tasked with preempting foreign threats and protecting the country’s interests through wetwork operations, including assassination of anyone who could be a threat to South Korean (SK) national security. Since its founding in 1976, during the rule of Park Chung-hee, the agency’s very existence has remained a state secret from everyone, including the President.
After entering NSS, the pair learned that Seung-hee was their undercover scout agent. While the duo was surprised and somewhat hurt, Hyun-jun was angered the most as he felt his heart was toyed with. During a team celebration dinner, Hyun-jun confronted her alone and the two embraced the passion they have for one another. Afterward, the two maintained a secret romantic relationship without the knowledge of anyone.
Hyun-jun is then sent off on a solo mission to Hungary. In Hungary, Hyun-jun accomplishes his mission and while escaping, becomes wounded. Shortly later, Hyun-jun finds out that he was betrayed by fellow agent Sa-woo. Seung-hee attempts to help Hyun-jun escape, but a car explosion separates the two and both are misled to believe that the other person are dead. Hyun-jun is then saved by an unknown voice and learns of the secret society “IRIS”.
One year later, Hyun-jun returns to Korea to seek revenge. During this time, North and South Korea are set to reunify. However, the members of IRIS are intent on stopping the reunification. They attacked a local mall for hostages to blackmail both sides of the Korean government to threaten SK’s president into announcing their political stand to not unite with NK. Hyun-jun became the negotiator for the president Cho Myung-ho (Lee Jung-gil) and for the hostages’ release. Eventually, Sa-woo was killed. With all things hindering the reunification process gone, the peace talks resumed.
At the reunification summit, the president personally requested Hyun-jun to be at his side. Concurrently, NSS agents realized they’ve been duped: the presumed dead IRIS agents at the mall crisis was actually the hostages. The true IRIS agents escaped and infiltrated the summit to kill all delegates. IRIS agents killed and replaced various security agents for a massacre. Unfortunately, it failed with Seung-hee’s interference. She pretended to be an IRIS agent and used the confusion to stop them. This created mass panic as IRIS agents make their stand against all security forces. Ultimately, the SK president and NK forces survived the assault and Hyun-jun saved the day again.
In the aftermath, Hyun-jun and Seung-hee left NSS for a normal life. While Seung-hee waited at a lighthouse for Hyun-jun, he was driving up to meet her and propose. However, during the drive, Hyun-jun was shot in the head by a sniper. He dies while looking at Seung-hee at a distance away at the lighthouse, unbeknownst to her.
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