The best part of FFXIII by far was Lightning, the game's main character. Unfortunately she seemed to all but disappear in the game's sequel, being instead relegated to a pay-for DLC content. She's been sleeping it seems, and is about to wake up and return with a vengeance. It's not FFXIII-III, it's Lightning Returns: Final Fantasy XIII.
Lightning awakes 500 years in the future, having witnessed a vision about the end of the world. She has 13 days to rewrite destiny or life as they know it will perish. Seems to me they are taking the name of the game literally: She returns, and there is a final event happening in 13 days.
No pressure, eh? While the dialog in this trailer seems a little overwritten, that's pretty standard for this set of Final Fantasy games. How all these characters are still alive after five centuries is a bigger mystery to me than how the world will end, but I have yet to finish FFXIII-2 so that might be on me.
Lightning Returns: FXIII will be available on the PlayStation 3 and Xbox 360 sometime this Fall.