Story:
"San Andreas,
Dr. Brandon Wilson has created a dangerous virus who can transform the Human to, a Zombie
who kills all the people that he found. Frank Tenpenny, a cop who wanted to own
San Andreas with the help of Mike Toreno, a govrement agency gave him alot of money so they can use the virus.
Now the whole San Andreas is under infection, Zombies are all around
Some survivors are left, trying to escape from the nightmare
Some will make, some will not"

Go to this topic in GtaForums to know more informations:
http://gtaforums.com/topic/696012-gta-zombieland-mission-pack/