Hyun-jun (Lee Byung-hun) and Sa-woo (Jung Jun-ho) are important members of the NSS (National Security Service). They both fall in love with their beautiful boss Seung-hee (Kim Tae-hee), but the latter is in love with Hyun-jun. Deputy Director Baeksan sent them to rescue the scientist, who entrusted a USB flash drive to Hyun-jun before his death. Baeksan secretly sent Hyun-jun to assassinate North Korea's No. 2 figure. Hyun-jun fulfilled his mission, but was killed. Out of brotherhood, Sa-woo released Hyun-jun. The latter encountered an explosion when he was about to flee with Seung-hee. Hyun-jun fled to Japan and reconciled with the North Korean agent Sun-hwa (Kim So-yeon) who was tracking him, and they moved forward and backward together. Seung-hee, who escaped from death, firmly believed that Hyun-jun was alive and violated regulations for many investigations. Hyun-jun was hunted down in Japan, causing the tragic death of his savior's family. Hyun-jun, who was desperate, defected to North Korea and attacked the NSS headquarters with a terrorist organization. He met Seung-hee, whom he had been thinking about day and night, but still could not recognize her. Xianjun finally solved the secret of the USB flash drive. It turned out that Baishan was the murderer of his father, and there was a mysterious organization behind him - IRIS. The high-level talks between North and South Korea, the captain of the guard conspired to create a terrorist explosion. Shengxi was captured alive by terrorists during the investigation, and his lover turned out to be his enemy. What greeted them was an even more cruel conspiracy... This film is the first spy war masterpiece created by KBS with an investment of 20 billion won, and it has repeatedly set ratings records.