The game is FreeSpace 2 by Volition, Inc. The year is 2367, thirty-two years after the "Great War." (Which Great War was that? You mean the one against those scary cat-people? Oh, sorry, wrong space-combat game - that was the superb series of Wing Commander games and the abysmal movie adaptation thereof.) The Terrans (that means you, Earthling) are still stranded countless light-years away from Earth, with slim hopes of ever returning to the fertile mud-ball they once called home. For the time being, the Terran forces have occupied themselves by fending off bandits and rogue Terrans, who you learn to take VERY little guff from early on in the game. See one of the first missions, "Surrender Belisarius!" for a taste of the tolerance your outfit has for traitors. Soon however, you stumble across a star-gate of ancient and unfamiliar design and realize that it just might be your ticket home. The only problem - somebody's already using it, and it doesn't look like they want to share.
So who is it that comes streaming out of the portal hell-bent on the utter destruction of the human race, mom, and apple pie? That's right, your old enemies, big rubber cat-puppets! No, wait - that was that monstrosity of a movie based on some other space-combat game. Once again, the bloodthirsty Shivans are back to slaughter Terrans and Vasuvans alike, but this time they mean business. The Vasuvans are your alien allies, who you killed a lot of in the first game, only to join forces with to combat the more powerful Shivans. The Shivans are a bit more mysterious - we're not treated to a glimpse of them, at least not early on in the game, but I bet they're not gonna end up being lame-o puppets. As it turns out, the hordes of nigh-unstoppable warships you had to trash in the last game were just the Shivans' version of U.N. peacekeeping force. Now they're armed to the teeth and they won't rest until you're sucking vacuum.