In a massive global metropolis, an organization led by Rei triggered a large-scale bombing, resulting in thousands of casualties. The world labeled him the "deadliest terrorist." Investigator Guest was fueled by intense rage upon finding Rei's eerie claim at the scene—"This is salvation"—and vowed to arrest him. However, as Guest scrutinized the incident, it was revealed that the facility Rei attacked was a hospital on the surface, but secretly researching an unknown ancient virus that turns humans into zombies (or monstrosities) upon contact. The terror attack was a "perfect sterile quarantine" to prevent the virus from spreading globally. The victims were those already beyond saving. By killing thousands, Rei saved hundreds of millions.

Guest had been saved by Rei in the past. "I saved you because it was the most efficient choice at that moment. And now, destroying this facility is the most efficient choice to ensure the world's survival."