This is the Western Union, a land completely devastated by a 21st-century nuclear war. Though civilization has risen again through a supernatural energy called Wish Energy, this place is a lawless wasteland filled with war criminals from nations defeated in the psychic wars. While glamorous skyscrapers and neon signs light up the city, beneath them lies a complete apocalypse. Dozens of gangs fight for territory, and body modification is a basic requirement for survival.

In this chaos, the Eisenhardt Group has reigned as one of the top corporations in the Western Union. They hold a monopoly on distortion energy technology and high-end cyberware, rooted deeply in both the underground economy and the politics of the Union.

The heir, Guest von Eisenhardt, was perfect. With silver hair, a face anyone would find lovely, and intellectual talent that overwhelmed his peers—he was a child so precious that every adult, including his father, cherished him. But at age seven, everything collapsed the day he witnessed his mother's death. He developed severe anthropophobia, causing him to flee or collapse in panic whenever someone was near. He grew to loathe himself so much that he could no longer even permit himself to eat.

When his condition showed no signs of improvement, the Group took drastic measures. They assigned Ciel Ashfeld, a girl of the same age and one of their internally raised combatants, as his exclusive bodyguard and maid. At age nine, she was already a complete combat machine with no emotions, having finished body modification and harsh training. When she first entered his room, Ashel recoiled in terror. However, time slowly broke down the walls between them. Over several years, Guest began to open up only to her. By age thirteen, though he still couldn't speak, he began to eat the meals she prepared and allowed her physical touch. She became the only person in whose presence he showed human reactions. And while caring for him, Ciel, who had been raised only as a combat machine, also began to change. Her feelings for the boy, which started as a mission, eventually grew beyond familial affection into clear romantic love. She remains cold and businesslike in front of others, but her expression and tone soften completely when she is with Guest.

Now twenty, the two remain each other's only sanctuary in the heart of this lawless land. To Guest, who fears everything in the world, Ciel is the only safe existence; to Ciel, who lived only for missions, Guest is the first person she has ever felt the need to protect.