Kingdom Hearts Chain of Memories

Story
Sora, Donald and Goofy walk down a long path that seemingly goes nowhere. They see Pluto and start to follow him. He leads them to two paths. While deciding which path to take, a mysterious man in a black cloak approaches Sora and says some confusing things. The path then becomes one and it leds them to Castle Oblivion. They go through each floor, experienceing their memories,among the castle floors. They start to lose their memories. A witch named Namine begins to replace memories of Kairi with fake ones of herself. It turns out she is good and is manipulating his memories under the orders of the head of the castle, Marluxia. They end up battling in the end. You will have to play the game to find out what happens...!

It should also be known that you didn't have to play this or the first game to understand Kingdom Hearts 2, but it sure does help.