I don’t know much about making stories…but maybe you could use the mechanics to help you. If your players get to high enough levels, killing zombies won’t net any exp, but they may be granted some for the methods they use kill them. You could include some important characters that aren’t zombies, but something undead, that they would have to get to, to stop shit. It would be kinda cool if some ancient demon from the seventh circle of Hell was responsible for this, and your guys would have to travel down there to stop everything. But of course, hordes of zombies would ever be present, and creative ways in which to destroy them would be the key to getting ahead. Maybe you could include those ways as part of the story. Depends on what kinda flair you want your plot to have. I can’t help much beyond suggesting stuff that’s already normal zombie apocalypse fanfare, but you gotta start that shit in a ruined Waterdeep. That would rock. Get the players a real enough setting before they eventually travel down to Hell itself. Hope this helps a little bit.