Three hundred years have passed since the holy knight Al was bitten by an undead creature and died, only to awaken as a mindless monster. Against all logic, he retained his sanity as his flesh slowly rotted away, leaving him a living skeleton who wanders a world overrun by the very horrors he once swore to destroy. He is discovered by Astrantia, a descendant of a witch, who makes him a proposition. In exchange for becoming her knight and helping her end the curse that plagues the land, she will use her powers to restore his lost body. This agreement leads the pair to enroll in an academy designed to train saints, specialized teams composed of holy knights and holy women who fight to eradicate the undead. Al must conceal his true nature as a skeleton within the very institution dedicated to destroying his kind, relying on the centuries of combat experience that have made him an exceptionally powerful warrior.

The world is one where humanity struggles against hordes of monsters and the undead. The primary human defense force is built around these pairings of holy knights and their holy women masters. Al and Astrantia, having already achieved notable success by defeating a powerful creature known as the Pestilent Guardian Dragon, are granted the prestigious title of White, signifying their elevated rank and effectiveness. Al, despite his undead transformation and membership in a group called the Twelve Corpses, remains a holy knight at heart and continues his fight to protect people.

Their journey becomes more complicated with the introduction of the Twelve Saints, an organization that includes other holy knight and holy woman pairs. Al and Astrantia encounter Theophilus, a fellow member of the Twelve Corpses, and his holy woman, Nemophila. A strange dynamic immediately presents itself. While Theophilus is utterly obedient to Nemophila, she despises him and directly asks Al to kill her own holy knight. This request forces Al and Astrantia to navigate a treacherous plot while they are simultaneously assigned a mission to eliminate another powerful member of the Twelve Corpses, a foe known as the One-Armed Giant.

The central narrative arc follows Al and Astrantia as they work together to achieve their mutual goal of ending the curse. A major throughline is Al's struggle to maintain his secret identity at the academy, balancing his need to hide his skeletal form with the necessity of using his immense strength to survive and protect his master. As the story progresses into subsequent arcs, they face the political and social complexities of the Twelve Saints, dealing with other teams whose motivations are not always aligned with their own. The evolving relationship between Al, an undead who clings to his humanity, and Astrantia, a descendant of witches with her own goals, forms the emotional core of the story as they confront external monster threats and internal treachery alike.