CoM also explains a lot. ;) Overview of Chain of Memories-
Sora, Donald and Goofy are lead into Castle Oblivion by a mysterious figure. Inside, they begin to lose their memories, and Sora begins to remember about a girl who lived with him on Destiny Islands. In the end, they discover that Namine was being forced to change Sora's memories so that Marluxia and Larxene could take over Organization XIII. Zexion and Lexaeus, two more members of the Organization, hear of what Marluxia and Larxene are planning. They decide to do the same, except instead munipulate Riku (he and King Mickey both somehow turned up in Castle Oblivion). Vexen (another member) fights Riku and creates the Riku Replica, who's sole purpose in his mind is to protect Namine (who Sora wants to find). Axel, the final member of Organization to be in Castle Oblivion, isn't really on Zexion or Marluxia's side. Vexen acts as though he is on Marluxia's side, but is found out, and Axel kills him (to proove who's side he is on). As Riku and Sora both make their way through Castle Oblivion, they kill Lexaeus and Larxene, and the Riku Replica stops fighting Sora. Axel frees Namine, and that recks Marluxia's plans, so the two face off. Marluxia escapes though, and Axel is left to fight Sora. Sora wins, and Axel seems to die, but really he just warps away. Sora then catches up with Marluxia and kills him, and then Namine puts his, Donald and Goofy's memories back. Thats the last thing those three do before KH2. Riku then gets to Zexion, who tries one last time to munipulate him. He fails though, and Riku defeats Zexion (who escapes). Zexion would have lived through the events had Axel not told the Riku Replica to kill him (which is a shame, because Zexion was the coolest member of Organization XIII. None of the ones I've seen in KH2 look as cool as him). Axel is the only one of the six members to survive the events in Castle Oblivion. :)