"Genma Wars: The Eve of Myth" is a manga series that blends elements of historical fiction, supernatural intrigue, and political drama. Set in a fictionalized version of Japan during the early 20th century, the story revolves around the emergence of ancient, malevolent spirits known as Genma, who seek to manipulate human affairs and plunge the world into chaos. These entities, born from humanity's darkest impulses, infiltrate the highest echelons of power, influencing key figures in government, military, and society.

The protagonist, a young and idealistic military officer named Ryuuma Nagare, becomes entangled in the conflict when he encounters a mysterious woman named Shizuru, who possesses the ability to sense and combat the Genma. Shizuru is revealed to be part of an ancient lineage tasked with protecting humanity from these supernatural threats. Together, Ryuuma and Shizuru form an uneasy alliance, navigating a treacherous landscape of espionage, betrayal, and supernatural warfare. Their mission is to uncover the Genma's ultimate goal—a ritual known as the "Eve of Myth," which, if completed, would grant the Genma dominion over the mortal world.

As the story progresses, Ryuuma and Shizuru are joined by a diverse cast of characters, each with their own motivations and connections to the Genma. Among them is Kagetora, a rogue warrior with a personal vendetta against the spirits, and Akira, a brilliant but morally ambiguous scientist whose experiments inadvertently aid the Genma's plans. The narrative explores themes of loyalty, sacrifice, and the blurred line between good and evil, as the characters grapple with their own inner demons while battling the external threat.

The plot is driven by a series of escalating confrontations between the protagonists and the Genma, culminating in a climactic struggle to prevent the completion of the Eve of Myth ritual. The manga delves into the historical and cultural context of its setting, incorporating elements of Japanese folklore and mythology to enrich its supernatural elements. The interplay between human ambition and supernatural manipulation serves as a central tension, highlighting the fragility of societal structures in the face of otherworldly forces.