Set in Universal Century 0123, the armed group Crossbone Vanguard, founded by Meitzer Ronah and preaching “Cosmo Aristocratism,” suddenly attacks the space colony Frontier IV, pushing it into open conflict with the Earth Federation Forces. The battle throws the colony into chaos, and Seabook Arno and his friends—caught in the middle during a school festival—struggle to reach safety as war erupts around them.