In the distant future of the NCE 0120's, a down-on-their-luck Martian mercenary gets wrapped up in a very great deal of political and extraterrestrial violence. SV, you are this gun-for-hire, and occasionally a very broad cast of anime characters dealing with the problems of the future: colonialism, energy crisis, human trafficking, fascism, and refrigerator freeze rays.
Weary of the endless cycle of reincarnation and battling against the forces of light, a demon queen accepts an offer from the destined hero... to attempt a quiet life in the Hero's hometown, avoiding battles for the fate of the world, her own irritating minions, discovery by the church, and all manner of things which could interfere with her retirement.
Things don't go to plan.