This is from my review of the whole game...
Zombie Island of Dr. Ned first off offers something that I found an amazing addition to setting, trees and greenery. The addition adds on an end of game side story that has Dr. Ned creating zombies that take over a Jacobs’ weapons factory that is based there. So the purpose is to find out what happened on the island and stop the nefarious Dr. Ned and his army of zombies. The enemy additions are of course zombies and lots of them. There is also a mission called “Brains” that I implore you seek out first because it requires you to collect zombie brains and I found and did this quest after I finished the main mission and it was brutally boring so if you want the achievement find the mission to spare yourself the boredom. All in all The Zombie Island of Dr. Ned is a fine addition which brings about new and varied areas to explore and is worth the price.