Game starts at the end of Ground Zeroes.
You are left behind on a sinking motherbase, expecting to die.
Then a wormhole opens and you jump in.
You find yourself in a post apocalyptic place, everything is destroyed around you, desert etc.
There are the zombie kind of enemies and a huge one that patrols the map. Thats the final boss that you need to take out.
There are some people that help you and you start a base.
For the most part you believe you are in a parallel universe but you discover you are in the future and if you don't kill that thing this is fhe future of earth.
It has 2 endings and one of them is fun:
At the end if you jump back in a wormhole, there is a cutscene that shows you alone, picking up a photo and wandering off.
The game implies you are one of the soldiers Snake has been collecting during the side missions

I believe it was a nice touch.