Kim Hyun-jon and Chin Sa-u are inseparable friends. After entering the special forces, they both come under the watchful eye of the secret organization NSS, whose main goal is the reunification of the Korean Peninsula. Having undergone training and becoming NSS agents, the friends unexpectedly meet Choi Seon-hee, the girl whom each of them had seen only once, but for both it was love at first sight. The young people begin their spy lives, experiencing the happiness of love and the pangs of jealousy. They do not yet know that somewhere in the United States there lies a hostile NSS group of killers called Iris, which keeps them all in its sights.