In the year of R.C. (Redo Century) 916... The Sils, a race of winged folk had passed their days in peace, hidden away from the world of man. Then the Empire appeared without warning, and their long, unbroken peace was suddenly placed in peril. Neun, a Sils boy, flees the village with his childhood friend Zehn. They found themselves chased by Imperial soldiers and in the end, Zehn was struck by a strange illness she obtained from shielding Neun: An ailment called "Doll Sickness". Neun continued to flee with Zehn until they met Umikaze Kei, a member of the Resistance Army who fights against the Imperial Empire. To save Zehn, Neun resolves to enlist in the Resistance Army's intelligence corps. And so, Neun marches to the front lines in search for more information on Doll Sickness..