Long ago, alien lifeforms arrived on Earth and fused with the planet's inhabitants. Because of their appearance, they were nicknamed "Mecha-ude," since the fusion resulted in a striking resemblance to mechanized limbs. Through an involuntary partnership, ordinary middle schooler Hikaru Amatsuga works alongside Arma, a Mecha-ude who is an extremely rare and special existence. To protect Arma, the resistance group ARMS assigns Aki Murasame, a dual Mecha-ude user, to follow him wherever he goes. However, she is also searching for a mysterious Snake-Type Mecha-ude wielder, who has left many of her comrades in a critical state. Behind the scenes, an organization known as the Kagami Group is hunting for the mythical "Trigger Arm," a Mecha-ude said to grant unlimited power. When Aki's life is threatened by this group, Hikaru must team up with Arma to find the courage to fight back and, along the way, learn that his life will never be ordinary again.