The events of the first game, Descent: Freespace, culminate with the destruction of the Sol jumpnode. So the GTVA, and the millions and billions of humans that are part of it, are cut off from Earth. There's a sequel there, but Volition isn't the developer to make it. In fact, it's already been made.
Blue Planet is a mod that takes up that mantel. It tells the story of the return to Earth, and extrapolates from the hinted Hindu overtones of naming a destructive, implacable race 'Shivans'. It might seem odd for a space sim to get into questions of godhood and religion (although less so after Battlestar Galactica), but this is space opera, and it's nothing if not known for tackling macro-ideas.
Through some freak of subspace travel, the fleet of ships tasked with making first contact with Earth are diverted into an alternate universe, where the first Great War didn't go so great for Earth, instead leaving it a charred, volcanic mess. Humanity all but wiped out, the campaign follows the exploits of Commander Bei, exploring ideas of transcendence and reincarnation while at the same time letting you shoot lots and lots of spaceships, with ever more powerful and satisfying weaponry.
It's just as much of an homage to Freespace 2 as it is its own narrative, with nebulas making a welcome return, while some of the greatest set-pieces receive nods and re-imaginings. It's not until about halfway through, with the introduction of the Vishnans, a 'maintainer' race, that Blue Planet starts to come into its own, presenting a far larger set of themes and ideas than Freespace 2 exhibited.
There's less of the military commentary, but at the same time it provides a much grander story, with the displaced GTVA fleet aiding humanity's survivors while trying to find a way home. Naturally, they do find their way back to their reality, but it's hardly to the welcome they're were expecting to give the Sol System...
That's Age of Aquarius, the first part of The Blue Planet mod. The second part, War In Heaven, details the conflict between the GTVA and the Sol forces, putting you in the shoes of a rookie pilot on the Earth side, faced with killing pilots who are similarly in the dark as to why they're even fighting the battle. Neither side is right, and the constant, unrelenting reinforcement of that fact in the mission briefings and radio chatter makes things... uncomfortable, to say the least.
It's the other side of the coin, focusing on the ambiguity and blind nature with which orders are followed that was so successfully touched on in the Freespace 2 campaign, where Age of Aquarius so excellently embellished the awe and power of the game's more dramatic moments. By separating them they're allowed to be much more deeply investigated, framed in different conflicts so as not to bleed into one another.
This is supposed to be a retrospective, looking back at a game that was brilliant through a nostalgia-tinted gaze, but Freespace 2 isn't an old game. It was old, over a decade ago, kicked out the door and forgotten weeks later like most game releases, but that's not the same game you can play today. It still costs the same as a 10-year-old game, but development never stopped.
Sure, it changed hands, moved from a dedicated team to anyone who wanted a look in, but the things that have been done are testimony to the fact that passion for a universe and a game can get you a hell of a long way. Freespace 2 isn't as good as it's ever been - it's better. Much, much better.