The group of survivors in the basement of the building includes a mix of people from various backgrounds, including a single mother with her young son, an elderly couple, a former soldier, a nurse, and a college student. They work together to ration their food and water and to barricade the door from the infected outside.
As time goes on, tensions rise among the group as they begin to realize that rescue is unlikely, and they may be stuck in the basement for the rest of their lives. They also face the ethical dilemma of whether or not to try to save any of the infected who they come into contact with.
Meanwhile, outside the fenced-off area, society begins to break down as the virus spreads and survivors resort to looting and violence to survive. The group in the basement must also deal with external threats, including looters and other survivors who try to break into their building.
As the days pass, the group starts to lose hope of ever being rescued and must confront their own mortality. Some members become increasingly desperate and begin to turn on each other, leading to a thrilling and devastating finale.