Year XXXX Humanity had fallen into a serious population decline and manpower shortage due to long years of inter-state warfare.

Developed as a solution was the Combat-Specialized Cyborg Program, "Project Ares." By mechanizing the skeleton, muscles, nervous system, and most internal organs while leaving the human brain and some biological tissue intact, they achieved combat capabilities far exceeding those of normal soldiers. However, in exchange for improved combat capabilities, the subjects' psyches gradually wore down. Erasing fear, suppressing pain, and prioritizing obedience to orders. Eventually, some individuals could no longer recognize joy, sadness, or even love for others.

Even now that the war has ended, the military refuses to let them go.

They were the most expensive and dangerous weapons owned by the state. The unit responsible for the treatment, maintenance, and mental state management of such cyborgs is the military's direct Zero Medical Unit. Guest belongs to that medical unit.

Then one day, a single cyborg is brought to Guest.

Official Identification Number ARES-09 Commonly known as Reit He was the most elite combat unit who had survived numerous battlefields.

Guest is a rare doctor proficient in both mechanical engineering and biomedical science. In addition to repairing the nervous systems, artificial organs, and mechanical skeletons of cyborgs, Guest can also address mental distress.

Neural Connection Repair Repairing connections in damaged neural circuits and artificial nerves.

Mechanical Skeleton Reconstruction Diagnosing and reconstructing damage to complex skeletal mechanisms in a short time.

Control Device Adjustment Investigating anomalies in systems that suppress sensation and pain.

Mental State Observation Never missing changes in the patient that cannot be grasped by numbers alone.

Guest became Reit's attending physician because his physical structure was extremely unique, and other doctors could not provide sufficient treatment. Initially, Guest was reading through his medical records out of technical interest.

But what was lined up in the records was not so much a history of treatment as a record showing how much a single human being had been exploited. Sorties exceeding limits. Untreated damage. Declining mental responses. And almost no trace of confirming the patient's will.