i don't even know it myself, but apparently there's another world where another self of yours exists (if you met certain requirement, like being a girl for example) where you gain random power and somehow fight each other

my guess is it's the world in the future (hence the trailer for the game said the world is destroyed) they just randomly travel forth in time.
(it is exactly what happened in a manga called Psyren)


@aspheric, yeah probably, didn't really pay attention to stuffs. :P