In a medieval fantasy world where heroism has been twisted into the ultimate punishment, the worst criminals are not executed but sentenced to become Penal Heroes. Forced to fight on the front lines against the Demon Blight, a corrupting contagion that rots the land and transforms creatures into violent monsters, these conscripts are denied even the release of death. If a hero falls in battle, they are forcibly resurrected to continue their never-ending war, losing fragments of their memories and identity each time they return.

The story follows Xylo Forbartz, a bitter and battle-weary fighter who leads Penal Hero Unit 9004. Once a respected captain of the Holy Knights who served the Goddess Senerva, Xylo was convicted of the unthinkable crime of killing his own deity. During a failed mission, Senerva became corrupted by the Demon Blight, threatening to transform into a Demon Lord. Xylo claims he performed a mercy killing at her request to prevent a greater catastrophe, but the church denied this account, labeling it blasphemy. For this act, he was stripped of his rank, executed by decapitation, and reborn as a Penal Hero with a magical seal on his neck that will detonate if he disobeys orders or attempts to flee.

His unit comprises an assembly of dangerous and eccentric outcasts. Dotta Luzulas is a compulsive thief with hundreds of thefts to his name. Venetim Leopool is a master conman who once incited a civil war through fraud. Tatsuya is an enigmatic veteran who may be the sole surviving hero from the first war against the Demon Lords, having been resurrected so many times he barely remembers his original self. Tsav, Norgalle Senridge, and others round out the unit, each with their own dark secrets and specialized combat skills. Despite their criminal backgrounds, the unit functions as a surprisingly cohesive and effective fighting force, though they face constant scorn and mistrust from the very people they protect.

During a mission, Xylo encounters Teoritta, a mysterious and powerful childlike Goddess of Swords sealed within a coffin. She possesses immense strength and can summon blades at will. Recognizing the potential to change his circumstances, Xylo forms a pact with her. In exchange for her cooperation, Teoritta demands that once every enemy is defeated, he must shower her with praise and pat her head. This alliance gives Xylo new power but also draws him deeper into a maelstrom of political intrigue and divine conspiracies.

As the series progresses, Xylo and Unit 9004 undertake increasingly dangerous missions against the Demon Blight, from rescuing miners abandoned by the church to defending fortresses against overwhelming hordes. The narrative reveals that the hero system is even more sinister than it first appears. The Goddess who resurrects the heroes is running out of power, and each revival chips away at the heroes' sense of self, with some having already lost so much of their original personalities that they are barely themselves anymore. There are hints that the unit was assembled deliberately by a figure named Kaufzen to create a force capable of achieving results without interference from obstructive bureaucrats, and that some members may not be what they seem—including one who might secretly be a rogue Demon Lord.

The central conflict expands beyond mere survival as Xylo seeks revenge against those who framed him and uncovers a conspiracy reaching the highest levels of the kingdom. The political machinations of the military and the Temple become increasingly prominent, with the Penal Heroes caught between their duty to fight the Demon Blight and their desire to expose the corrupt system that condemns them. By the end of the first major story arc, Unit 9004 has become an unexpected symbol of hope for the common people, even as the authorities continue to view them as disposable tools. The stage is set for a rebellion that could shatter the chains of eternal punishment and redefine what it truly means to be a hero.