In Kingdom Hearts, there is a poem which describes the storyline. You can find it if you don't press anything at the start menu for about half a minute, or simply read it from below.

Thinking of you, wherever you are,

We pray for our sorrows to end,
and hope that our hearts will blend.
Now I will step forward to realise this wish.

And who knows:
starting a new journey may not be so hard
or maybe it has already begun.

There are many worlds,
but they share the same sky-
one sky, one destiny.