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Sora and his buddy Riku are chilling with their mutual friend Kairi when their home is attacked by the Heartless, and they are seperated. At the same time, Donal and Goofy find out that King Mickey is leaving on a journey to save the Kingdom's Worlds, and he tells the pair to find the boy who holds the key. Coincidentally enough, Sora is mysteriously dumped into Traverse Town with a Keyblade in his hand. He meets Donald and Goofy, and the trio zip around to Disney-based worlds in order to lock the keyholes(which prevents the Heartless from attacking.)
At some point, Sora finds out that the Disney villian Maleficent(from Sleeping Beauty) has manipulated Riku into becoming one of her minons. In fact, Riku has become possessed by Ansem ( A king who went crazy and became the leader of the Heartless), and Sora ends up fighting Ansem at the End of the World (where worlds that have been conquered by the Heartless die.) After Ansem is defeated, the door to the final world, the Kingom Hearts ( the heart of all worlds), is opened. Mickey and Riku suddenly appear, step inside the door to Kingdom Hearts, and close it behind them, vanishing from Sora.
The GBA game, Kingdom Hearts: Chain of Memories, starts off where KH ended. Sora is told to journey to Castle Oblivion, but along the way he runs into the Organization. The Organization uses a girl named Namine to erase Sora's memories, and two members (Marluxia and Larxene) decide to use Sora to eliminate the other members and take control themselves. However reheaed Axel (who's shown in KH2 screens) takes out out the traitors and disappereas. Sora fought Marluxia and came out victorious, he meets Namine and saves her from the Organization; she tells him that in order to get his memories back, he'll need to forget about her and everything that happened in Castle Oblivion. The game ends with Sora and his friends asleep in a giant flowerlike doohickey.