In this game, we continue soon after the previous game ends, still in that field. Sora awakens at night and takes a walk, finding a man in a black hood, who speaks barely a word, and merely walks off. In the following days, Sora, Donald, and Goofy walk along that path to a large castle, Castle Oblivion.
Castle Oblivion is a stronghold of the mysterious Organization XIII. While not very well-known during the course of the game, they are definitely a vile force that must be dealt with.
As they ascend through the thirteen floors of the castle, the trio find that their memories are being messed with. They also enccounter various members of the Organization and end up defeating them. I know I’m really summing this up quite shortly, but that’s what this game mostly is.
Eventually, at the end, Sora and Naminé meet up. Naminé is a person connected to Sora and Kairi, for Naminé is Kairi’s Nobody. (A Nobody is the husk of a person, left without a heart.
When a person is turned into a Heartless, as Sora was while having Kairi’s heart within him in Kingdom Hearts I, and they are of a strong heart, their body begins to act on its own.)
Naminé apologizes for messing with Sora’s memories, but it was not of her own choice: the Organization forced her to. They agree that Sora, Donald, and Goofy should sleep for the next year, hidden in another world, while Naminé fixes their memories…