ok so...spoilers for all of you!
sora is walking with goofy and donald and he sees a castle and is all "hey, maybe riku's in there." so they go in and there's this guy in a cloak and the guy in the cloak is all " hey, the further you get in the castle...the more you forget." and sora is all.."what?" but then sora runs around the castle and worlds based on his memories...and then this girl that looks like kairi shows up and messes themm up....and riku's all "DARKNESS" and sora forgets everything except the memories that the girl has written into his mind and then she decides to return them and he goes into a little pod thing to return them and..........are you confused yet? hahaha ok just go here and scroll down to 'plot and setting' for a good explanation
:
http://www.reference.com/search?q=chain%20of%20memoriesriku's story is quite cool...
"So don't run from the light — and don't fear the darkness.
You'll be able to escape the deepest darkness —
Riku: and I'll be able to see through the brightest light..."
end spoilers.