Happiness

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; Sae-bom's neighbours are suspicious of her; Tae-Seok and Yi-hyun team up to investigate the mysterious disease.

Sae-bom is suspicious of Tae-seok; 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.

The sudden lockdown puts Park Seo-yoon in Sae-bom's care; 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.

Joo-hyung visits the gym with a plan to get back at the couple in unit 501; a person is let into the building while another heads out.