Taking place shortly after the war of the last alliance, the return of sauron has you take control of many different factions attempting to sieze control of middle earth. But they are all being controlled by Sauron the Great, who is ever eager to return to mordor. Some factions will fall, and others will survive, but in the end only Sauron can rise to power again. Take part in events such as the fall of moria, and the kin-strife of gondor, and the rise of the witch king.
However, this campaign has to be broken apart into the campaign and an expansion, so this file isn't the whole story.