In the world of the Eterna Empire, humanity is hemmed in by magical beasts that have plagued its borders for a thousand years. The only force capable of standing against these monsters are the Machine Gods known as Unbreakers, colossal mecha owned by the six great noble houses. Each Unbreaker draws its power from the life force of its contracted pilot, and only the head of a house is permitted to pilot one. The throne of the emperor itself is decided through a brutal succession war in which these houses and their Machine Gods clash.

Dusty Miller is a commoner who serves as the butler of the Endeavor house. His master, Cesius Endeavor, is the current pilot of the Unbreaker called Type: Shadow and a candidate for the next emperor. Dusty has lived alongside Cesius and his sister Aura since childhood, bound by loyalty and a quiet promise. He has no right to pilot a Machine God and expects none, content to protect his master from the shadows. His skill in combat has earned him the epithet the Death Butler, as he has eliminated many who sought to steal the Unbreaker.

Everything changes when a branch family of the Endeavors unleashes a coordinated attack using controlled magical beasts. Dusty is gravely wounded, losing an arm and the use of his legs. In the chaos, the dormant Unbreaker activates on its own, and Cesius makes a fateful decision. He transfers the contract to Dusty, making the butler the new pilot of Type: Shadow. The Machine God transforms from a skeletal form into a heroic white, black, and gold machine, granting Dusty the power to manipulate gravity with his left hand. He becomes a full participant in the battle for the imperial throne.

The Unbreaker itself carries a dark secret. Its power consumes the pilot's life span. A special skill called the Final Skill burns a decade of life in a single use, and the Machine God houses the souls of previous pilots as navigational AIs. The AI assigned to Dusty is a long-haired woman named Fujinoha, who speaks with a sharp tongue and repeatedly urges him to break the contract. Despite the danger, Dusty refuses to abandon his master and the dream the three of them share: that Cesius will become emperor, marry Aura, and the three will live together as a true family.

Dusty's first major confrontation is against Victor Ironheart VIII, the head of the Ironheart house and a rival for the throne. Victor pilots Type: Knight, a sword-wielding Machine God with teleportation abilities. Victor looks down on Dusty as a commoner and is obsessed with Aura. Their battle is fierce, with Dusty using his gravity manipulation to counter Victor's sword strikes. In the end, Dusty activates the Final Skill, crushing Victor with a giant shadow hand, and wins. Victor, humbled and forced to confront his own loneliness, acknowledges Dusty as his friend.

The story follows Dusty as he navigates the politics of the six great houses, fights magical beasts and rival Machine Gods, and uncovers more about the true nature of the Unbreaker system. The Alv tribe, an elf-like race with the ability to control magical beasts, becomes a factor in the larger conflict. The succession war accelerates into a full battle royale, with Dusty, Cesius, Aura, and the loyal staff of the Endeavor household standing together against enemies from within and outside the empire. Themes of loyalty, class division, the cost of power, and the meaning of family run through the narrative.