In Daikuuriku, everyone is born female and once they reach the age of 17, they have the choice to determine their gender. In order to safeguard the serene theocratic society of Simulacrum, magical aero machines called "Simoun" are utilized which can only be operated by young, undecided girls. The Simoun have the power to activate "Ri Maajon," a sorcery that can vanquish vast numbers of adversaries at once. Upon discovering Simulacrum’s secrets, industrialized nation Argentum launches an invasion, leading to a lopsided battle with the priestesses, known as the Simoun "Sibyllae," who pilot the Simoun. In order to continue flying in the midst of war, the Sibyllae are permitted to defer their decision on their permanent gender. Aaeru, a resolute young Sibylla with an enigmatic past and Neviril, the leader of Chor Tempest, Simulacrum's elite Simoun squadron, must guide their fellow priestesses through internal conflicts and battle in the sky.
