Set four thousand years before the rise of the Empire in the film series, Knights of the Old Republic depicts a "more civilized age" for the galaxy. Unfortunately a Sith Lord named Darth Malak is leading a vast army of Sith against the Republic and especially any Jedi that happen to get in the way of his conquest. The game begins with the player's character waking up on a Republic ship that has fallen under attack by the Sith. And what happens from that moment on is entirely up to you: the choices you make during the game affects pretty much everything else that follows afterward, right up to which ending you get. Play the straight and narrow and you'll wind up a hero of the Republic, while yielding to temptation will lead to the Dark Side and all the power and glory and pleasures befitting the fallen.
Long story short: it's a very good game that's more than worth the purchase price. And hopefully LucasArts will also have Knights of the Old Republic II for sale on Steam sooner than later. So what are you waiting for? Buy it today, ya meatbags!