begins on Destiny Islands, where Sora, Riku, and Kairi live. The three friends want to leave the islands to explore new worlds and have prepared a raft for this purpose. One night, the islands are attacked by darkness and shadow creatures—the Heartless. Sora seeks out his friends, finding Riku, who disappears into darkness. At the same time, Sora obtains the Keyblade. The islands are presumed as destroyed, and Sora is left adrift. Meanwhile, King Mickey has left his world to deal with the increasing darkness and left instructions for mage Donald and knight Goofy to find the "key" that will save the King and the Worlds.
Donald and Goofy use the Gummi Ship to travel to Traverse Town, to where Sora has drifted. Sora encounters the Heartless again, and meets Leon, who explains the Heartless are beings that consume hearts, and that the Keyblade is the only weapon capable of defeating them. A man named Ansem, the leader of Leon's home world, is said to have studied the Heartless. Sora meets Donald and Goofy and the three decide to travel together; Donald and Goofy to find Mickey, and Sora to find Kairi and Riku. The three go to various worlds based on Disney films, finding that the Keyblade also locks "Keyholes", passages the Heartless use to take the heart of a world. A group of Disney villains, led by Maleficent, seek out the seven Princesses of Heart to unlock the Keyhole that leads to Kingdom Hearts, a repository of knowledge and power and the source of all hearts. This group includes Riku, whom Maleficent promises she will help in finding Kairi. Maleficent sows distrust in Riku, telling him Sora has abandoned him and Kairi for new friends and the Keyblade. An increasingly antagonistic Riku finds Kairi's body, but cannot find her heart.
Sora and his friends eventually arrive at Hollow Bastion, the homeworld of Ansem and the headquarters of Maleficent. Riku takes the Keyblade from Sora, revealing him as the true Keyblade master and that Sora only received it in his absence. Donald and Goofy, taking their order to follow the "key" seriously, leave with Riku. Sora challenges Riku, stating his heart derives strength from his friends; his friends return to him, as does the Keyblade. In his shame, Riku meets a cloaked man who goads him to give into the darkness. The three engage and kill Maleficent, but then meet a strangely-behaved Riku with a Keyblade that unlocks hearts. Sora finds Kairi's body and Riku, who reveals himself to be Ansem, possessing Riku's body. Ansem explains that Kairi is a Princess of Heart and her heart was trapped within Sora's body since Destiny Islands were destroyed. After defeating Ansem, Sora uses Ansem's Keyblade to unlock his heart by impaling himself, releasing both his and Kairi's heart. Kairi's heart returns to her body, in turn completing the final Keyhole, while Sora becomes a Heartless. Sora is restored to human form by Kairi and resolves to confront Ansem.
Ansem is found in End of the World, the combined remnants of worlds taken by the Heartless. Ansem explains his belief that the nature of the heart revolves around darkness, and he seeks Kingdom Hearts, as that would be the ultimate darkness. However, upon opening the door to Kingdom Hearts, it reveals its light, overwhelming and destroying Ansem. Beyond the door are King Mickey and Riku, and they help Sora and the others close the door, as there are many Heartless beyond it. Mickey and Sora use their Keyblades to lock the door. The worlds lost to the Heartless reconstruct themselves, separating Sora and Kairi. The game concludes as Sora, Donald, and Goofy resolve to find Riku and Mickey
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