The story is set in Nagasaki prefecture in Minami Shimabara city, the fabled origin of the Shimabara Rebellion that sent shockwaves through all of Japan in the early Edo period. It is also where a third-year junior high school student named Shirou spends his days with his school friends. One summer day, Shirou's older childhood friend Akane returns to Minami Shimabara from Tokyo. Shirou and Akane take a stroll around Minami Shimabara with another longtime friend, Tokiya. While these friends are dealing with nostalgia and more complex feelings, a mysterious creature named Goron suddenly appears before them. This encounter with a mere little creature causes great events to unfold once more in this city... (Source: ANN)

Long ago, alien lifeforms came to Earth, fusing with people on the planet. Due to their appearance, they were dubbed "Mecha-ude," as when fused, they bore a striking resemblance to mechanized limbs. Through an involuntary partnership, average middle schooler Hikaru Amatsuga works alongside the Mecha-ude Arma, who is an extremely rare and special existence. To protect Arma, the resistance group ARMS assigns the dual Mecha-ude user Aki Murasame to follow him wherever he goes. However, she is also looking for a mysterious Snake-Type Mecha-ude wielder, who has put many of her comrades in a critical state. Behind the scenes, an organization known as the Kagami Group is searching 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. [Written by MAL Rewrite]