In ancient times, extraterrestrial beings descended upon Earth, merging with its inhabitants and forming a unique bond. These beings, known as "Mecha-ude," manifested as mechanical limbs when fused with humans.
Caught in an involuntary alliance, Hikaru Amatsuga, an ordinary middle school student, finds himself partnered with the rare and exceptional Mecha-ude named Arma. To safeguard Arma, the resistance group ARMS assigns Aki Murasame, a dual Mecha-ude user, to accompany and protect Hikaru. Simultaneously, Aki is on a quest to locate a mysterious Snake-Type Mecha-ude wielder, responsible for gravely injuring her comrades.
Operating behind the scenes, an organization called the Kagami Group seeks the legendary "Trigger Arm," a Mecha-ude rumored to bestow limitless power. When Aki's life is endangered by this group, Hikaru must join forces with Arma to gather the courage to fight back. Along their journey, Hikaru realizes that his existence will never be ordinary again.