Son ami du lycée, le détective Jung Yi Hyun, appelle Sae Bom pour l'informer qu'une recrue du SOU, Lee Jong Tae, est impliquée dans une affaire de meurtre qui vient de se produire. Il semble que Lee Jong Tae ait donné une sorte de drogue à son ami qui a mordu le cou de la victime et l'a tué. Sae Bom, qui doit travailler dur et gagner des points pour avoir droit à un appartement de location publique, va chercher Lee Jong Tae malgré l'avertissement de Yi Hyun.

Sae Bom devient enfin propriétaire de sa maison. Au terme de son dur labeur, elle pense que le bonheur est à portée de main. Cependant, elle finit par apprendre qu'elle a peut-être attrapé une nouvelle contagion, qui pousse les personnes infectées à mordre les gens après avoir perdu la raison. Bien que Sae Bom et Seung Young aient prêté serment de garder le secret, des journalistes les approchent avec une grosse somme d'argent. Pendant ce temps, Yi Hyun et Tae Seok du Central Disaster Countermeasures Headquarters (CDCH) unissent leurs forces pour découvrir la véritable nature de la maladie.

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.

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.

The residents in Building 101 suspect Sae Bom and Yi Hyun are infected and quarantine them in their unit. With them quarantined, the residents discuss what to do about them. Making matters worse is the dwindling food and water supply. If they can't get it from outside, the only choice is to take it from someone else. Meanwhile, Sae Bom and Yi Hyun figure out how to find out who is infected.

Kim Se Hoon, the guy on the 15th floor, goes missing. Sae Bom and Yi Hyun try to figure out why he disappeared and what he was trying to hide. The residents in the building flock to his unit and squabble over the supplies he had stashed. Meanwhile, Yi Hyun contends with his thirst and keeps Sae Bom from knowing about his infection.

Sae Bom and Yi Hyun fall into a crisis due to the resident representative, Oh Ju Hyung, and other residents conspiring with Andrew. Tae Seok learns that Sae Bom is in danger and tries to get her out. And Yi Hyun, who still hasn't told Sae Bom about being infected, tries to protect her any way he can. However, they end up face to face with a mob of the infected.

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.