Basicly Sora goes to castle oblivion in search of Riku and King Mickey.
A man in a black rain coat(part of the orginization) seems to guide him there.
The guy in the rain coat says"In order to gain something you must lose something"(something along that line.
As sora ventures thru Castle oblivion he slowly starts losing his memories, but starts to remember deep memories.
This introduces Namine'.
Sora 'remembers' a promise he made to her to keep her safe.
Soon he forgets all about Kairi
Then the Riku replica comes in to play, he has the same memories as sora, they start to fight blah blah blah
Then When sora get to the end of the castle he finds out that his memories are fake, he never actually met namine' before until then, and the real Riku was not that fake one that he kept fighting with.
Namine' trys to help sora get his memory back, and puts him in a bubble thing
Then thats where kh2 starts i'm guessing
Tell me if i missed something