Kingdom Hearts is the original game in the series. Revolving around a fourteen-year old boy named Sora, it details his adventures after his homeworld is destroyed by sinister creatures known as the Heartless, and his attempts to restore his world and reunite with his friends. In the process, he meets many characters from classic Disney films and a handful from the Final Fantasy video game series.
Kingdom Hearts begins with Sora's inner Awakening, which foreshadows Sora's destiny, but soon progresses to Destiny Islands, where Sora lives with his two best friends, Riku and Kairi. The three friends seek 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 heartless. Soon after, Sora obtains a mysterious weapon, the Keyblade, to defend himself. Using it to fend off the creatures, then islands are soon destroyed, and Sora is left adrift, Riku and Kairi's whereabouts unknown.
Meanwhile, Mickey Mouse (The King) has left his world to deal with the increasing power of darkness and left instructions for mage Donald Duck and knight Goofy to find the "key". Donald and Goofy use a Gummi Ship to travel to Traverse Town, where it happens Sora has also drifted to. Sora encounters the creatures again in the town, and eventually meets Leon, a mysterious swordsman who explains they are Heartless, creatures that consume hearts, and that the Keyblade is the only weapon capable of defeating them. The king of Leon's homeworld, a man named Ansem, is said to have studied the Heartless. Soon after, Sora meets Donald and Goofy rather hastily, and the three work together to take down a large Heartless. From there, they decide to travel together: Donald and Goofy to find Mickey, and Sora to find Kairi and Riku.
The three travel to various worlds based on Disney films, finding that the Keyblade also locks "Keyholes", passages that the Heartless use to take the heart of the world. A group of 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 now includes Riku, who is swayed by Maleficent's promises that she will help find Kairi. At the same time, Maleficent sows distrust in Riku, telling him that Sora has abandoned him and Kairi for new friends and the Keyblade. An increasingly antagonistic Riku manages to find Kairi's body, but her heart is missing.
Sora and his friends eventually arrive at Hollow Bastion, the home world of Ansem and the headquarters of Maleficent. Riku takes the Keyblade from Sora, claiming that he was fated to have it all along and Sora was simply the "delivery boy". Donald and Goofy, taking their order to follow the "key" seriously, reluctantly leave with Riku. Sora calls on his courage and enters the Bastion anyway, then challenges Riku again, stating that his heart derives strength from his friends. His friends return to him, as does the Keyblade. Shamed, Riku flees and meets a cloaked man who goads him to give into the darkness.
Sora, Donald, and Goofy, meanwhile, engage and destroy Maleficent. They soon after meet a strangely-behaving Riku with a new Keyblade that unlocks hearts. He leads them to Kairi's living but unfeeling body; Riku then reveals himself as completely possessed by Ansem. The newly reformed Ansem explains that Kairi is the last Princess of Heart, and that her missing heart has been trapped within Sora's body since the destruction of Destiny Islands. Spurred to action, Sora defeats Ansem; however, he cannot seal Hollow Bastion's keyhole because Kairi's heart is still in his body, thus the keyhole remains incomplete. Sora uses Ansem's Keyblade to unlock his heart, releasing both his and Kairi's heart, but turning him into a Heartless. Kairi's heart returns to her body, in turn completing the final Keyhole; she then returns Sora to human form by the strength of her heart. The group resolves to follow and end Ansem's plan.
Ansem retreats to the End of the World, the combined fragments of worlds taken by the Heartless. Upon being found and finally beaten, he explains his belief that darkness is the heart's true essence, and he seeks Kingdom Hearts, the source of all hearts, and therefore the ultimate source of darkness. However, upon opening the door to Kingdom Hearts, it reveals Light, killing Ansem. Beyond the door are Mickey and Riku, back in control of his own body. They help Sora and the others close the door, as there are many Heartless beyond it, but Riku and Mickey must remain inside to help seal it. Mickey and Sora then use their Keyblades to lock the door. The destroyed worlds reconstruct themselves; Kairi is pulled back to the Destiny Islands, but Sora promises before separating they will all reunite some day. Sora, Donald, and Goofy resolve to find Riku and Mickey, though they are unsure of where to start. Luckily, Pluto appears, clutching a letter from Mickey in his mouth; he runs off with Sora, Donald, and Goofy in tow, ready for a new set of adventures. The silent narrator from the beginning of the game states in the final scene that Sora's destiny is to open the door to light.