Back Arrow takes place in Lingalind, a continent surrounded by a wall that delivers mysterious Rakuho capsules, containing mechanical armbands known as Briheights, to various locations each month. These armbands enable the wearer to transform into a mechanical being, fueling the continent's constant turmoil as nations vie for control. A Rakuho crash-lands in the countryside, carrying an unexpected passenger, a black-haired man claiming to be from beyond the wall, and identifying himself as Back Arrow.