Hyunjun and Sawoo, close friends and rivals from the South Korean special forces, are recruited by the secret agency NSS. They both fall for Seunghee, the beautiful but lethal profiling specialist at NSS. When they are faced against each other in a nuclear terrorist attack, they are forced to make their final decision and Seunghee's formidable secret is revealed.

Set 3 years after the first season, members of the National Security Service (NSS) team, including Jung Yoo Gun, Ji Soo Yeon and other agents attempt to pull the curtain on mysterious group IRIS. Agent Yoo Gun is the leader of the NSS TF-A team. While he goes up against Baek San, Yoo Gun becomes involved in a case which changes his fate forever.