Sorry, I might not have phrased it clearly enough; it didn't happen in Nemesis, it happened after Nemesis in the original timeline. This game happens even later than that, in the early 25th century. The fall of the Romulan Empire followed by a Klingon invasion of the Gorn led to an end to the alliance between the Klingons and the Federation, and eventually open warfare between the two powers. In the game you can play as a Klingon or a Federation officer, and there's a lot of player-vs-player content that I haven't gotten into yet.
Because of that, there's a lot of combat in this game, but you'll also get diplomatic, exploration and scientific missions (at least if you play Federation like I did). Combat is all action-based, with space combat being similar to a lot of previous PC Star Trek games, such as the Starfleet Command series, and ground combat being essentially a third-person shooter with the new control scheme.