300 years after the Calamity War's end.

The powers which ruled the Earth Sphere have been overthrown, giving way to new rulers of a new world, exhausted of battle. Meanwhile, far away on Mars, the sparks which threaten to ignite a new war have been struck...

Our protagonist Mikazuki Augus, a member of the private security firm Chryse Guard Security (CGS), takes on a mission as bodyguard of Kudelia Aina Bernstein, a young girl who agitates for Chryse's independence from Earth. However, when the elite Gjallarhorn unit, charged with suppressing revolutionaries, raids CGS, its higher-ups deliberately use Mikazuki and the other boys as a decoy to cover their own retreat. Orga Itsuka, leader of the boys' squad, takes this abandonment by the adults as motivation to launch a full-fledged coup d'état. With orders from Orga to hold back Gjallarhorn, Mikazuki sorties in Calamity War-era scrap that CGS had been using as a generator—the Gundam Barbatos...

(Source: Gundam.info)