Years ago, extraterrestrial beings made contact with Earth and interbred with humans, resulting in the creation of a new species known as "Mecha-ude" due to their mechanical characteristics. Unexpectedly, Hikaru Amatsuga, an ordinary middle schooler, finds himself bound to a rare and extraordinary Mecha-ude named Arma, attracting the attention of the rebellion group ARMS. To keep them safe, Aki Murasame, a dual Mecha-ude wielder, is assigned to protect them from imminent danger. However, Aki has her eyes set on finding the Snake-Type Mecha-ude's mysterious holder who caused harm to her team. Meanwhile, the powerful Kagami Group is searching for the legendary "Trigger Arm," a Mecha-ude that supposedly provides immense abilities. As the Kagami Group threatens Aki's safety, Hikaru and Arma team up to protect themselves and realize that their ordinary lives will never be the same.