Her friend from high school, Detective Jung Yi Hyun calls Sae Bom to let her know that a SOU recruit, Lee Jong Tae is involved in a murder case that just occurred. It seems Lee Jong Tae gave some sort of drug to his friend who bit off the victim’s neck and killed him. Sae Bom, who needs to work hard and earn points to qualify for the public rental apartment, goes out to get Lee Jong Tae despite Yi Hyun’s warning.

Sae Bom thinks Tae Seok is hiding something and feels uneasy about it. The new apartment is peaceful compared to the unrealistic dormitory. Sae Bom and Yi Hyun return to their everyday lives. Although it doesn't seem right, they try their best to adapt. However, late in the night, they hear a constant banging from the unit above them.

The apartment complex is now without electricity. Sae Bom and Yi Hyun find traces of an infected person inside Building 101. Who could be infected? The residents suspect others of being infected and wait for the lockdown to end. Meanwhile, Tae Seok makes a decision knowing that the antibodies may be inside Sae Bom.

Sae Bom learns that happiness is not your house but the person you are beside. And that person to her is Yi Hyun. She determines to make it back to the apartment, her house, and to where Yi Hyun would be. Dying or getting infected doesn't scare her as she tells him to wait for her.