The series is set in the years 2119–2121 AD, several decades after humanity ("Terrans") has expanded its presence beyond Earth, terraforming and colonizing Venus and Mars. These three planets are "the Homeworlds", the core first of the Terran interplanetary state and later of Neosapien Commonwealth. Not all Terrans are affiliated with the Homeworlds, however: the Pirate Clans, descendants of Terran criminals exiled to the Outer Planets who live off looted Homeworlds' space freighters, are a major independent faction in the show. The first episode opens with the Earth Congress dispatching the entire Exofleet, humanity's space-based military, to counter the Pirate threat.
With war with the Pirate Clans looming, an uprising begins among the Neosapiens, an artificial humanoid race coexisting with Terrans. In the back-story, the Neosapiens were used primarily as slaves during the colonization of Mars and Venus and therefore have been engineered to be physically stronger and better adapted to hostile environments than humans. Their mistreatment by Terrans led to the First Neosapien Revolt fifty years before the series' begin, which was mercilessly crushed but had brought some positive changes into their lives. Still not content with his fate, the Neosapien Governor of Mars, Phaeton, sets a new insurrection, codenamed "Operation [Neosapien] Destiny", in motion as soon as the Exofleet leaves to chase after the Pirate Clans. The absence of the Exofleet is also a part of Phaeton's plan as it enables the Neosapiens' capture of the Homeworlds without much effort.