Best friends who are like brothers Kim Hyun-jun and Jin Sa-woo train under the south korean army's 707th special mission battalion until they are scouted by NSS (National Security Service). The two were independently scouted by the beautiful NSS agent, Choi Seung-hee.
Unaware they have been scouted, the two are secretly captured and subjected to a simulated torture test. Both passed the test and are deemed qualified to become NSS agents. In the process, they both fall in love with Seung-hee, who has kept the scouting process a secret from them.
After completing the test, Hyun-jun and Sa-woo are introduced to the director of the NSS, Baek San.Baek San explains 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 learn that Seung-hee had been their undercover scout agent. While both were surprised and somewhat hurt, Hyun-jun is angered the most as he feels his heart was toyed with. During a team celebration dinner, Hyun-jun confronts her alone and the two embraced the passion they have for one another. Afterward, the two maintain a secret romantic relationship.
Hyun-jun is then sent off on a solo mission to Hungary. In Hungary, Hyun-jun accomplishes his mission and while escaping, is 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 into believing that the other is 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 preventing the reunification. They attack a local mall and take hostages so that they can blackmail both sides of the Korean government and to threaten SK's president into announcing their political stand to not unite with NK. Hyun-jun becomes the negotiator for the president Cho Myung-ho
and for the hostages' release. Eventually, Sa-woo is killed. With the obstacles to the reunification process removed, the peace talks resume.
At the reunification summit, the president personally requests Hyun-jun to be at his side. Concurrently, NSS agents realize they have been duped: the presumed dead IRIS agents at the mall crisis were actually the hostages. The true IRIS agents escaped and infiltrated the summit to kill all delegates. IRIS agents kill and replace various security agents for a massacre. Fortunately, it fails due to Seung-hee's intervention. She pretends to be an IRIS agent and uses the confusion to stop them. This creates mass panic as IRIS agents make their stand against all security forces. Ultimately, the SK president and NK forces survive the assault and Hyun-jun saves the day again.
In the aftermath, Hyun-jun and Seung-hee leave NSS for a normal life. While Seung-hee waits at a lighthouse for Hyun-jun, he drives up to meet her and propose. However, during the drive, Hyun-jun is shot in the head by a sniper. He dies while looking at Seung-hee at a distance away at the lighthouse, unbeknownst to her.