On the Earth-esque planet of Daikūriku, everyone is born female. The theocratic nation of Simulacrum has a monopoly on the helical motor technology and as a result grew to prosperity. In Simulacrum, the girls grow up until age of seventeen, when they make a pilgrimage to a holy place known as “the Spring” to select their permanent sex.
Two other nations, Argentum and Plumbum, wage war against it in an attempt to steal the technology. It is defended by advanced airships called “Simoun,” which are piloted by priestesses known as sibyllae; a girl who has not yet chosen a permanent sex.
The sibyllae can inscribe enormous glyphs known as Ri Mājon in the sky using the Simoun to produce powerful magical effects both for combat and ceremony.
Aer is recruited to be a Simoun pilot after a terrifying attack by an enemy nation decimates the squadron known as Chor Tempest. To earn her wings she needs to find her way into the heart of Neviril, Regina of Chor Tempest. But Neviril’s heart still belongs to her previous Pair, lost in the battle when she attempted a forbidden Simoun maneuver.