Kingdom Hearts 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.^[1] One night, the islands are attacked by darkness and shadow creatures.^[11] Sora seeks out his friends, finding Riku, who disappears into darkness. At the same time, Sora obtains the Keyblade. The islands are destroyed, and Sora is left adrift. Meanwhile, King Mickey Mouse has left his world to deal with the increasing darkness and left instructions for mage Donald Duck and knight Goofy to find the "key".^[23]

Donald and Goofy use a Gummi Ship to travel to Traverse Town, to where Sora has drifted. Sora encounters the shadow creatures again, and meets Leon, who explains the shadow creatures are Heartless, beings that consume hearts, and that the Keyblade is the only weapon capable of defeating them.^[29] 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 that 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,^[10] 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 that Sora has abandoned him and Kairi for new friends and the Keyblade.^[30] 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, claiming that he was simply the "delivery boy".^[31] Donald and Goofy, taking their order to follow the "key" seriously, leave with Riku. Sora challenges Riku, stating that his heart derives strength from his friends; his friends return to him, as does the Keyblade. Shamed, 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.^[32] Ansem explains that Kairi is a Princess of Heart and that her heart was trapped within Sora's body since the Destiny Islands were destroyed. After defeating Ansem, Sora uses Ansem's Keyblade to unlock his heart (he impales 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 he resolves to follow Ansem.

Ansem is found in End of the World, the combined remnants of worlds taken by the Heartless.^[18] Ansem explains his belief that darkness is the heart's true essence, and he seeks Kingdom Hearts, as that would be the ultimate darkness. However, upon opening the door to Kingdom Hearts, it reveals its light, killing Ansem. Beyond the door are 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 reconstruct themselves, separating Sora and Kairi. The game concludes as Sora, Donald, and Goofy resolve to find Riku and Mickey.

Characters

Because Kingdom Hearts was a collaboration between Disney and Squaresoft, it features a mixture of familiar Disney and Square characters, as well as several new characters designed and created by the director, Tetsuya Nomura.^[21] The primary protagonist of the game is Sora, who is chosen to wield the Keyblade, a weapon for battling darkness. The game also features two of his friends, Kairi and Riku. For most of the game, Sora is joined by Donald Duck and Goofy. Donald, the court wizard, and Goofy, captain of the royal guard,^[22] were sent from Disney Castle to find the Keyblade.^[23] The three join forces to search for King Mickey Mouse, Kairi, and Riku. The primary antagonist is Ansem, who is seeking to find power and knowledge by using dark beings called the Heartless. The Heartless, hearts corrupted by darkness, serve as most of the enemies encountered in the game and come in a variety of shapes and sizes.

As a game meant to explore the fictional universes of various Disney films, there are over 100 Disney characters.^[24][25] While many serve as major characters in the story, others appear in cameo roles, such as the One Hundred and One Dalmatians puppies playing a part in a side-quest. Most worlds also feature a Disney villain whom the player must defeat. The player can summon various Disney characters to fight alongside Sora in battle, causing Donald and Goofy to withdraw from the battlefield for the duration of the summon. Available summoned characters include the Genie from Aladdin, Tinkerbell from Peter Pan, and Simba from The Lion King.^[13]

Square also incorporated several of its famous characters from the Final Fantasy series into the game, though the characters were tweaked slightly for novelty and to fit the game's back-story. On Destiny Islands, the player meets younger versions of Tidus, Wakka (both from Final Fantasy X), and Selphie Tilmitt (from Final Fantasy VIII). In Traverse Town, the player encounters Squall Leonhart (known in the game as "Leon") from Final Fantasy VIII as well as Aerith Gainsborough, Cid Highwind, and Yuffie Kisaragi from Final Fantasy VII.^[13] Rikku from Final Fantasy X and Irvine Kinneas from Final Fantasy VIII were both originally set to make cameo appearances, but were replaced by Yuffie and Wakka respectively.^[26][27] Cloud and Sephiroth (both from Final Fantasy VII) make appearances in Olympus Coliseum, where the player can fight them in the tournaments.^[13] The emphasis on characters from later Final Fantasy installments stems from Tetsuya Nomura's hesitation to use characters he did not design.^[28]

The game also uses other Final Fantasy icons. Moogles appear to provide item synthesis.^[1] Several weapons, such as "Lionheart" and "Save the Queen", share names with other weapons from previous entries in the Final Fantasy series. A name Riku suggests for the raft early in the game is Highwind, a reference to the airship of Final Fantasy VII. The magic-naming system in Kingdom Hearts (i.e. Fire, Fira, Firaga, etc.) is identical to Final Fantasy magic. The names of various spells are also the names of Gummi blocks, and various summons, weapons, bosses, and monsters are the names of Gummi ship blueprints