Usually the post apocalyptic stories take place in a far flung future after a cataclysmic event has destroyed most of the world, the economies have collapsed in on themselves, resources have become scarce, and its everyone for themselves. Usually the cause is a nuclear war leading to a radioactive fallout, or else is a mysterious event that acts as a McGuffin just to get the story rolling.
Usually there are cannibals and evil warlords. Sometimes vampires. Other times zombies. Every possible future scenario is different. Which is why it makes the post apocalyptic genre such a great sandbox to play in. I thought I'd take a stab at it.
Quick Synopsis: A scientist who has cracked the code of time travel, Dr. Eli J. Bannerman, develops a time travel device and sends himself back in time in order to save a dying planet--and consequently his dead wife, Maddy Emmerich. But unlike most time travel stories, where the hero prevails, Dr. Bannerman fails to alter the course of history enough to stop the impending death of the planet's ecosystem.
Upon returning to the future, he finds himself in a nightmarish world where a depraved mutant known as the Dark Lord Skaende Shendar Ontheiligen has taken over most of the world. Only a small rebel force, using guerrilla tactics, stands in the way of the Dark Lord's world domination. A rebellion headed by none other than Bannerman's supposedly dead wife (from an alternate timeline), Maddy Emmerich.
Bannerman, unable to return to the past, decides to help this world's version of Maddy Emmerich and joins the rebellion. However, the twist is in order to save this future from an evil tyrant, he might have no choice but to kill Maddy Emmerich.