Seven hundred years ago, half of humanity perished in the Great War in the Earth Sphere. Powerful weapons called Gundams, piloted by evolved humans called Newtypes, helped bring the war to an end, and became venerated as god-like guardians. Despite this, the Earth government considered Newtypes and Gundams to be a threat. Shortly after the war, the Newtypes were rounded up and banished to the space colony Cocytus, which was then gassed to kill its 10 million inhabitants and ejected from Earth's orbit.

Unknown to the Earth government, some of the inhabitants of Cocytus survived the gassing, and they and their descendants built a new civilization within the colony as it drifted through deep space. In the present day, inside Cocytus, the infamous assassin Bloody Kaoris is captured and sentenced to public execution in the Imperial City of Gun Dorado, but the authorities do not realize that this is exactly what she wants.

(Source: Gundam Wiki, edited)