Happiness

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 in any way he can. However, they end up face to face with a mob of the infected.

Sae-Bom escapes from the dormitory to rescue Yi-Hyun from the apartment. On reaching there, she learns what happened in the building in her absence. Colonel Han informs his team about a new hope residing in the apartment.