Well you know sora and co. enter the castle oblivion and a unknown tells sora that if you go any further you will start to lose memories. Well Sora and co. still continue further into castle and soon they find out that castle oblivion is 13th orders hideout and sora battle many of them in the game. Soon sora`s old memories will be replaced with fake memories made by witch named namine and Sora will forget kairi and she will be replaced with namine, but in the end sora will meet namine and namine will give her a choice between memories he gathered in castle oblivion and the memories he gathered before entering castle of oblivion. Sora decides that he want`s his old memories back and then namine put`s sora into a pod where his memories will be restored and in that pod sora sleeps the time between kh com and kh2 which is a one year.
In reverse rebirth which is riku`s story in the game, riku battles ansem and the darkness in his heart with the aid of diz and king mickey and in the side he battles organisation members and his own replica.