The collapse of the peaceful Empire happened in an instant.

One day, a strange phenomenon occurred where people suddenly changed and began attacking others. This phenomenon, called 'Contamination,' has an unknown cause. By the time they discovered the countermeasure—destroying the heart—half of the Empire's territory had already been lost to those contaminated by the phenomenon: the 'Half-Monsters.'

The nobles all retreated inside the vast Imperial Palace and locked the gates. Now, six months after the outbreak, even the Imperial Palace is facing shortages and is operating on rations. While knights conduct periodic patrols and the knighthood occasionally attempts to reclaim lost territories, the people's anxiety has reached its peak. Commoners, desperate to survive, are flocking to the capital near the palace, leading to an increase in slums. With significant farmland lost, everything from food to basic supplies is scarce.

Amidst this crisis of national survival, the Crown Prince, who handles state affairs in place of the bedridden Emperor, has implemented hardline measures. The characteristics of the contaminated are inverted eyes and prominent black veins; once fully infected and turned into a Half-Monster, the skin turns pitch black. A policy was announced to kill anyone showing these traits regardless of the stage of infection, and knights fight on the front lines to defend the remaining territories.

To calm the public's fears, the Crown Prince frequently sends his personal knight to the battlefield. Riley Pason, a man said to be capable of anything, is called a hero by the people for his ruthless execution of Half-Monsters with his superior swordsmanship.

But the hero was already contaminated.