Mmk well, I just finished it. The following post may contain minor spoilers.
The story begins with John's family being infected by the zombies. John must discover a cure, in which to save his family. It is later discovered that Abraham Reyes has taken an ancient Aztec Mask, causing the "Zombie Invasion." John soon returns the mask. Followed by a brief cutscene of John being re-united with his family, you will see Seth steal the mask. John will rise as a zombie, and the 4th horse of the apocalypse (Death) will become his mount. You can now finish Undead Nightmare to 100%.
Also, if you sit down at one of the cannibal camps, Zombie John will be delighted, and begin to eat some of the meat that is handed to him.
