The heroes that once saved this land have long since died. Now, the plague is back and those heroes are returning. Unfortunately, they're as restless spirits. A new generation of heroes will have to send them back to eternal rest along with the zombies walking the streets. That's where you find yourself in Plaguelands, a new scenario for Zombicide: Green Horde and No Rest for the Wicked that CMON's posted on their website.
From the post:
The land is infected again with the plague. The Necromancers are back to claim for themselves. The once fabled heroes are roaming around as restless spirts, and it’s up to us to end both the Necromancers and put the heroes to rest!