3-500 years in the future, Earth has expanded to colonize Mars and beyond the asteroid belt. Mars has its own government now, and they are antagonistic towards each other with constant saber rattling threatening war. It has a more advanced military but much smaller than Earth's. The outer planets are mostly colonized by each of these planets governments, and the citizenry are looked down upon like your typical third world country citizens who are barely scraping to get by from both Earth and Mars. Earthers, Martians, and belters are all physically and culturally different by this point, due to generations of living apart in different amounts of gravity and the different challenges associated with that.
The first book/season starts with alternating viewpoints from James Holden, a dishonorably discharged Earth Naval officer who now works on a ship that mines and hauls ice from Jupiters belt to different places throughout the outer planets, and Miller, who is a "Belter" detective working on a colony base on Ceres, an asteroid in the belt.