It's Christmas time in New York City. A strange virus has appeared in the city, turning seemingly normal people into insane, violent Infected. Rookie Officer Stevens is one of the first victims of the Infected. But unlike others, Stevens is stangely immune and doesn't lose control. The mysterious Dr. Schaeffer discovers that Stevens' immune blood can be used to destroy the otherwise unkillable Infected. Schaeffer creates a special "viral gun" that fills shells with blood from Stevens' arm and blasts them straight into the Infected, causing them to splat spectacularly.