In the world of Daikuuriku, everyone is born female and chooses which gender they wish to become at age 17. In this world, the peaceful Simulacrum theocracy is guarded by magical flying machines called "Simoun", which can only be flown by youngsters. girls who have not yet chosen a sex. The Simoun can activate a magical power known as "Ri Maajon" that can destroy a large number of enemies at once. When the industrialized nation of Argentum decides it needs to invade Simulacrum to acquire Simoun's secret, war breaks out, drawing the Simoun "Sibyllae" (priestesses who fly the Simoun) into an unequal battle. Because the war is raging, the Sibyllae have an exemption from choosing a permanent sex as long as they are willing to keep flying. Aaeru, a determined young Sibylla with a mysterious past, and Neviril, the leader of Chor Tempest, Simulacrum's most elite Simoun squad, must guide their fellow priestesses through conflicts both within their ranks and in the sky.