Sae-bom, a police officer, starts digging into a case to get points for qualifying for a public rental apartment. While investigating the murder case, Yi-hyun realises something unusual.

Sae-bom moves in with Yi-hyun but her neighbours are suspicious of her, and Tae-Seok and Yi-hyun team up to investigate the mysterious disease.

The news about the mysterious disease is revealed to the public. Sae-bom gets irritated by the banging noise from upstairs and heads there to complain.

Sae-bom is tasked with taking care of Park Seo-yoon due to an unexpected lockdown. Yi-hyun investigates the fitness instructor's connection to Next.

The whole complex gets infected, except for Building 101. Sae-bom and Yi-hyun struggle with the constant conflict between the renters and owners. A lack of supplies prompts the residents to step outside.

Yi-hyun and Sae-bom go up to the rooftop to make sure that the zombies cannot enter the building. Ju-hyung visits the gym with a plan to get back at the couple in unit 501.