It is the 28th century and ever since the invention of FTL travel humanity has spread to thousands of star systems. First few centuries of interstellar expansion were full of hope and optimism, but over time a kind of malaise set in. Maybe it was the disappointment that they didn't find gods, great truths, or some kind of bigger purpose on the way, but finally being among the stars left humanity feeling empty.
Depression and stagnation weakened the structures that had kept humanity together. Once powerful Earth Union -government found itself incapable of keeping civil unrest down as economy once powered by the rapid expansion became to shrunk. Corporate controlled cluster of systems formed the Federation seceded from the union leaving it even weaker. This led to a long civil war and the rise of the Collective.
All this left the area of space being settled over the depression years being of fairly little interest to anyone. Aside from few more populated planets here and there, the Frontier is mostly populated by less developed planets that are home to all kinds of communities, some space stations, mining operations and plenty of criminality. Being left mostly to their own vices for a century, Frontier people developed a strong sense of independence.
After the civil war Union attempted to reign in the frontier worlds, but this almost resulted in an uprising. As a compromise Union allowed the formation of Frontier Senate and gave it limited autonomy. Senate in turn established the order of Marshals, frontier born peacekeepers empowered by senate to enforce the law in their space and on planets and moons.
As humanity is finally mostly enjoying peace and the Frontier begins to turn less lawless it offers plenty of opportunities for all sides. Earth Union, Frontier Senate, Federation members, The Collective, criminal organizations, religious cults and plenty of others now have their interests set there and it's up to the order to protect the peace and quiet of common folk.