Glitch just proposed a topic for us to try out the site. I'm not going to reveal all his details (at least the ones he told me), but here's a basic run down.
We all arrive on Z through various ways. The mission/quest does not allow spaceships to land, so everyone coming in through space will have to portal in from an orbital space station or just portal in from another planet. I know this one applies to me - anyone with some sort of flight capabilities will be grounded for the story. It would be cheating for Copland to just be able to zip over the zombies on his board while the rest of the crew has to be on foot.
The basic story is that we all band together to retrieve some sort of item. We have to fight through the hordes of undead to recover it and then return to get paid.