Hikaru Amatsuga's peaceful routine is abruptly shattered when he saves Alma, a mechanical arm in distress. Alma merges with his jacket, pulling Hikaru into the realm of mechanical arms—enigmatic, conscious weapons that usually fuse with their wielders. Forced to escape, Hikaru is eventually seized by Aki Murasame, a twin‑arm practitioner employed by ARMS, the agency that defends machines from abuse.
The Kagami Group, a sinister outfit treating mechanical arms as disposable tools, hunts for Alma. ARMS labels Hikaru incompetent because he lacks an innate aptitude for arm wielding, so Aki is assigned to guard him. Yet Aki is overpowered by Jun Kagami, who mercilessly severs her two limbs, condemning the girl to a slow demise. Determined not to let Aki suffer that fate, Hikaru unexpectedly awakens his latent power and defeats Kagami. Now a full ARMS operative, Hikaru vows to uncover the secrets of mechanical arms and safeguard his new comrades. He commits himself to shield his newfound companions and allies from any threat. With unwavering resolve, he embraces the responsibility of safeguarding humanity's fragile balance against unseen dangers forever.
