Heian-kyō, capital and the center of elegant, aristocratic culture, is heavily guarded by a spiritual force field -- or so it seems. In reality, onmyōji (court magi who create the spiritual force field) can only defend the palace located in the northern part of the city; in downtown, monsters known as "horror" that feast upon human souls roam after sunset. There are, however, a group of heroes protecting commoners from "horrors" in darkness.

In a prosperous city named Russell City, an omen that threatens to shake its world begins to move within it. A man named Sword is the first to hear the first stirrings of the plot, and throws himself into a shadow war in order to expose it. His only clue is the key word "El Dorado." He meets Sophie, a woman searching for her missing older brother who has only left her with the same words: "El Dorado." With Sword having also lost his younger sister in the past, both are drawn together by the words, and work together to find out its meaning.