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 lock-down to end. Meanwhile, Tae Seok makes a decision knowing that the antibodies may be inside Sae Bom.