The story unfolds in Aionios, a world locked in endless war between the rival nations of Keves and Agnus. Their armies are made up of engineered soldiers granted only ten years of life, who fight and die to keep their colonies' Flame Clocks burning. Among them are off-seers, soldiers entrusted with playing flutes to send off the fallen. Two such off-seers stand at the heart of the tale: Noah of Keves and Mio of Agnus, each traveling with their own squad of comrades. When the two enemy groups clash and are then caught in a deadly attack, a mysterious power called Ouroboros binds their fates together. Forced to become allies and hunted by both nations, Noah, Mio, and their companions set out to uncover the truth behind the war that has consumed their world.