The end of the world has been and gone.
There was no one great natural disaster, no all-consuming world war, no catastrophic pandemic. Rather scores of storms, droughts and floods; dozens of vicious, selfish regional conflicts that only destroyed what could no longer be rebuilt. No single finishing stroke for Earth's great global human society, but you can still bleed to death from a thousand cuts.
The Red Planet fared better. Where Earth fell apart, Mars pulled together. Engineered men and beasts, aided by Bees, an outlawed distributed intelligence, survived through co-operation, because there was simply no alternative.
Fast forward to the present day. A signal - "For the sake of what once was. We beg you. Help." - reaches Mars.
How could they not help? A consortium of Martian work crews gather the resources for a mission: a triumphal return to the blue-green world of their ancestors.
And now here they are - three hundred million kilometres from home.
And it has all already gone horribly wrong.