On the hottest day of the year, an unknown virus spreads throughout inner-city Philadelphia. The infected victims, crazed by dehydration, begin attacking other residents of the neighborhood in gruesome ways. When the military is brought in to contain the situation, but realize they can't come up with a vaccine quickly enough, they fence off the area and let everyone die.  A group of locals, stuck in the basement of their building, behind the fences, and separated from their family members, band together to try to survive.