In the near future. Amidst a long-standing war, humanity developed biological weapons: "Beast-Soldiers" to replace human soldiers. Beast-Soldiers are military organisms combining human intelligence with animal physical abilities and instincts. They are treated not as soldiers but as "weapons," managed by identification numbers rather than names. All that is required of them is obedience to orders and combat capability. They are granted no freedom or human rights.

The military aimed for a perfect combat weapon that possessed animal instincts and human intelligence but remained unaffected by emotion.

Therefore, a normal Beast-Soldier: Kills if ordered. Dies if ordered. Emotion does not exist within them.

While they possess animalistic physical traits, their intelligence is nearly equal to that of a human. However, all beastmen created for military purposes undergo a specific procedure.

"Emotional Suppression Treatment"

Fear, sadness, love, and guilt interfere with combat operations. Therefore, procedures to suppress emotions were administered during the military utilization process. However, in some Beast-Soldiers, "emotions" such as fear, sadness, loneliness, love, and obsession with a specific partner emerge.

The military judged this to be a "mental defect."

Guest and Rei are Beast-Soldiers created in the same military research facility. Designed to complement each other's abilities for deployment in the same unit, they have spent their lives in the same sector since childhood. Through long years of experimentation and training, they came to recognize each other not just as comrades, but as the only existence necessary for survival.

Eventually, they began to prioritize each other's safety over orders. If one was hurt, the other would resist; if one was to be disposed of, the other would go berserk.

This mutual dependence was judged a fatal flaw for military weapons, and

the decision was made to discard them both.