When radiation seeped into the shelters, most society had crumbled. However, since then, extremely powerful individuals who have benefited immensely from positive mutations have arisen into powerful positions at the head of "networks". These individuals, known as "Bosses", have recognized their superiority and the potential of having underlings working for them. Some leverage these facts ruthlessly, seeking merely the expansion of their personal power, while others are more benevolent in their demeanor. Nonetheless, these networks are the formative elements of society on this new Earth. They aren't overarching as they carefully select new entrants to ensure good performance and utility (and in turn, perhaps without full knowledge, healthy offspring). This leaves a good proportion of the underground's population as wild scavengers who fend for themselves.
Just as the civilization left behind on Earth has begun to get itself back on its feet, the civilization that left it behind has returned, seeking to exploit the Earth once again. Such efforts endanger the networks, and while there is no coordinated resistance between them, nearly every network is offering its own, independent resistance.