For just 10 bucks (800 Microsoft points), Rockstar is adding what nearly amounts to a whole new game. There's a roughly six hour storyline, complete with cutscenes and new music. There are all new enemies, mounts to ride, outfits to wear, side-quests and challenges. There's even new multiplayer content (which I can't talk about until next week). This download is, to put it succinctly, absolutely huge. And it's all going to be available October 26.
Rather than continuing where Red Dead Redemption left off, Undead Nightmare is a side-story to the main campaign. Once you have it downloaded, you can select it from the main menu screen and hop straight into a tale of zombacalyptic proportions. This is a complete side-story -- everything you do in Undead Nightmare does not interact with your game progress in the main game.
The story goes like this: An infection is sweeping through the land, turning everyone into zombie shells of their former selves. As John Marston, it's up to you to discover the source of the outbreak and set things right. The brief bit of the actual storyline I played showed that Rockstar is clearly approaching this add-on with a sense of humor. The cutscene featured the old grave robber Seth -- as loony as ever -- trying to play poker with a zombified version of his old pal Moses. Things don't work out that well.