Hikaru Amatsuga's peaceful days come to an abrupt halt when he saves Alma, a mechanical arm crying out for help. Alma merges with his jacket, introducing Hikaru to the world of mechanical arms: mysterious, sentient weapons that typically become part of their users' bodies. Forced to flee for his life, Hikaru is eventually captured by Aki Murasame, a user of twin mechanical arms working for ARMS, an organization that protects these machines from unethical treatment. The Kagami Group, a sinister organization that treats mechanical arms as mere tools, seeks to recover Alma. Hikaru is considered a failure by ARMS due to his lack of natural aptitude for wielding a mechanical arm, so Aki is assigned as his bodyguard. However, Aki is defeated by Jun Kagami, who cruelly tears out her two mechanical limbs, condemning the girl to a slow death. Unwilling to leave Aki to her grim fate, Hikaru miraculously awakens his power and neutralizes Kagami. Now a full member of ARMS, Hikaru is determined to uncover the truth about mechanical arms and protect his newfound friends and allies.