We will start at the middle of the story, Los Santos is almost perfectly cleared. The protagonists, Androids, and several other figures will be introduced.
Worry not, as the story goes forward we will have a glimpse at the background and so - starting in the middle is merely for good introduction.



Who are they?! ANDROIDS.
What do they want?! COMPLETE ANNIHILATION OF ANY LIVING BEING

WARNING. You have just released 2 androids to freedom, accept? DECLINE DECLINE DECLINE
>>>Their release was accepted, in case of emergency use Emergency Deactivation Remote.
>>>>Emergency Deactivation Remote not found 404.
WAAHT NOW?! :panic:
Computer proceeds at annihilating the living of San Andreas.

They were programmed to destroy any human being on the lands of San Andreas, and so they did.
The powers the poccess are still unexplainable - but I assure you that will be unveiled as the story progresses forward.
;)