There was a great war called the "Calamity War". About 300 years have passed since the end of that war. The Earth Sphere lost its previous ruling institutions, and a new world was built under a new ruling system. A temporary peace came, and at the same time, a new war was ignited in the Martian Sphere far from the Earth. The protagonist, a young boy named Mikazuki Augus, accepted a mission to escort Kudelia Aina Bernstein, a girl who wanted to make the Martian city under the rule of Earth forces independent, from the private security company Chryse Guard Security (hereinafter referred to as CGS). However, CGS was attacked by the armed organization Gjallarhorn, which was trying to snatch the buds of rebellion, and retreated after using Mikazuki and other children as bait. The leader of the boys, Olga Itsuka, saw this as an opportunity and decided to raise the banner of rebellion against the adults who abused her. He gives Mikazuki the task of repelling Gjallarhorn, and Mikazuki enters battle using the Gundam Barbatos, a mobile suit from the Calamity War era which has been serving as CGS’s power source.