I have enjoyed Undead Nightmare alot, its almost as good as the Vannilla Game.
The game takes place in the same time, same map, and same places, but the setting has changed, (!Spolier Alert!) In the start of the Expansion your family gets attacked by "The Old Man" who has caught the plaque and turned into a zombie. You are not sure what is wrong with him, but then he bites your wife and son. They start to go crazy and you must hogtie them and restrain them in your home.
Marston then goes off to find a cure, and how this plaque was started.
His journeys take him threw New Austin, into Mexico. That is all i will tell you for the story.
-Save towns, as they will get attacked by zombies and are not safe till you clear all the zombies out
-Sasquatch (Thats all im going to say)
-Apocalypse horses (Set of 4.... Hey zombies are already here why not other horses.... P.S War is the best in my opinion.)
-New Events (Rather it is a courtisan that needs a ride to a safe town, or a person being chased by a horde of zombies, or a man being eaten... etc. There are plenty great Events that will occur)
-New Side Missions (There are many side missions, all of wich have something to do with Zombies)
-Zombie Horses! (Tho not as cool as the Apocalypse horses, there still cool..... Meet your new companion, a diseased horse.... P.S It never loses energy so you can run it as hard as you want, same with Apocalypse horses.)
-A Great Story line
-Comedy (You would think a zombie attack would make everything so dull, well lucky for you there is still that comedy during the stories.)
All horses, Except Apocalypse Horses, Start to turn left no matter what... This gets anoying because you have to keep it undercontrol or you might run into a pack of zombies, who will then yank you off your horse.... and most likely eat you.