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 finally gets to be a homeowner. At the end of all her hard work, she thought happiness was just around the corner. However, she ends up learning she might have caught a new contagion, one that causes the infected to bite people after losing their minds. Even though Sae Bom and Seung Young are under an oath of secrecy, reporters approach them with a hefty sum of money. Meanwhile, Yi Hyun and Tae Seok from the Central Disaster Countermeasures Headquarters (CDCH) join forces to find out the true nature of the disease.

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 gets walled up and quarantined due to Min Ji's mad person disease triggered by the Next drug. Whether they like it or not, the people living in the apartments have to stay within the walls together for a week. Sae Bom and Yi Hyun isolate the suspect Ju Hyung and provide shelter for outsiders. Meanwhile, during the joint investigation, Yi Hyun and Tae Seok discover the dealer of Next lives in Building 101.

All the apartments within the complex get infected, except Building 101. The phone lines and Internet get disconnected, and they are all on their own. Sae Bom and Yi Hyun struggle with the constant conflict between the renters and owners inside the building, including the people outside. Meanwhile, their food depletes, and they are left with only one option, to go outside.

After they return from their mission of obtaining food, the people inside won't believe anything they say about what they saw. Everyone starts to doubt each other, and distrust builds up among them as they hide their true feelings. Meanwhile, the personal trainer locked up in the fitness center turns into a monster, putting fear in the people of the building.

An infected person enters Building 101, and once again, Building 101 is under the threat of infection. Lieutenant Colonel Han enters the apartment complex with news to tell, but Sae Bom and Yi Hyun find him suspicious. A person goes missing due to the intrusion, and those with similar interests and minds stay close together.