In a government facility hidden from the eyes of the South Korean public, a top-secret project was underway. It was an experiment involving a new virus, and the children in the lab were the subjects. Most of the children injected with the virus were able to function normally without sleep, which was expected to have a significant impact on human development. However, some children were different. For months, they showed no symptoms, but then things began to turn strange. It started with mild cold symptoms and escalated to vomiting blood. By the time anyone realized the danger, the laboratory glass was already shattered and drenched in blood.

Due to its high infectivity—transmissible through mere contact with bodily fluids via a wound—and the government's clumsy response, the virus blanketed the entire Seoul area. The government belatedly locked down the capital region, but in the end, they could do nothing but trap the infected inside. The military units were merely pushing the people trapped in the capital into the swarms of the infected.