During fourth period, high schooler Da-jeong finds her classmate Tae-gyu dead in a classroom and Jeong-hun standing in front of him with blood on his clothes. In order to stop Jeong-hun from becoming the prime suspect, Da-jeong must help him find the real murderer, who is still inside the school building, under 40 minutes before period four ends.

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.

Eunha, who is a stateless person, is imprisoned for charged with murder. A warmhearted, coolheaded correctional officer goes through turbulent changes of his emotions after he meets the woman. The movie depicts the extraordinary love between the two in the limited space of a penitentiary facility.