This game is by far the best story written that shows the innevitable corruption of a protogonist... Best game ever...
The main fact is, War craft III story depends on just the same case. Nevertheless, this one is differentiating itself from the others ( starwars, lord of the rings etc) with its epic atmospehere. You will feel how your psychological and moral attitude differs through out the game outline.
The game makes you play all the races to reach the final in a connected relationship between all the events. Throughout the game you start with Arthas ( young prince of azeroth and son of king terenas, a paladin of light) and continue the game becoming a corrupted betrayer who kills his own father under the command of lich king.
At the final of Reign of Caos, the story obviously tell that "it is not over". Right after killing the evil, you are forced to plug-in the frozen throne expension to see the rest of the story.
I should say, Reign of caos and Frozen Throne are not two different games but they are one infact. So it is better to consider this two parts a main story.
Finally, Warcraft III is standing on the top of my Top 100 games ever list and obviously it will stand there for many years...