Long ago, extraterrestrial lifeforms arrived on Earth and merged with the human population. They were termed "Mecha-ude" due to their look, since when united, they bore a remarkable similarity to mechanized limbs. Hikaru Amatsuga, an average middle schooler, works with the Mecha-ude Arma, a highly uncommon and exceptional being, through an unwilling relationship. To safeguard Arma, the rebel group ARMS assigns Aki Murasame, a dual Mecha-ude user, to accompany him everywhere he goes. She is also on the hunt for a mystery Snake-Type Mecha-ude wielding who has left several of her friends in serious condition. Behind the scenes, the Kagami Group is on the hunt for the legendary "Trigger Arm," a Mecha-ude claimed to provide endless power.