In the world of Daikuuriku, all individuals are born female and select their desired gender at age 17. The peaceful theocracy of Simulacrum is defended by magical flying machines known as "Simoun," which can only be piloted by young girls who have not yet chosen a gender. These machines can activate a magical power called "Ri Maajon," capable of destroying large numbers of enemies simultaneously. When the industrialized nation of Argentum decides to invade Simulacrum to obtain the secret of the Simoun, war erupts, pulling the Simoun "Sibyllae" (priestesses who pilot the Simoun) into an uneven conflict. Due to the ongoing war, the Sibyllae are granted an exemption from choosing a permanent gender for as long as they continue to fly. Aaeru, a determined young Sibylla with a mysterious past, and Neviril, the leader of Chor Tempest, Simulacrum's most elite Simoun squadron, must guide their fellow priestesses through conflicts both within their ranks and in the sky.