any and all information is taken directly from the kingdom hearts wiki. i donot claim any of the information below as my own.
Plot
Kingdom Hearts begins with Sora’s Dive to the Heart, 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. They have even prepared a raft for this purpose, along with supplies for the trip. During this time, Sora meets a mysterious hooded figure who vanishes after giving Sora some cryptic words and a warning. One night, the islands are attacked by shadow creatures called Heartless. Sora seeks out his friends, finding Riku first, but Riku disappears into darkness, curious about what it contains. Soon after, Sora obtains a mysterious weapon called the Keyblade to defend himself with from the Heartless. Using it to fend off the creatures, he heads for the secret cave he encountered the hooded figure in, where he finds Kairi near the door. She turns to him, saying his name as the door behind her blows open. The ensuing blast of darkness sends Sora and Kairi both out of the cave. The islands are soon destroyed, and Sora is left adrift, with both Riku and Kairi’s whereabouts unknown.
Meanwhile, King Mickey has left his world of Disney Castle to deal with the increasing power of darkness and left instructions for court mage Donald Duck and Captain of the Royal Guard Goofy to find the “key”. Mickey’s wife, Queen Minnie, assigns Jiminy Cricket to journal Donald and Goofy’s mission. Donald and Goofy use a Gummi Ship to travel to Traverse Town, where it happens Sora has also drifted to after the destruction of Destiny Islands. Sora encounters the creatures yet 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 home world, a man named Ansem, is said to have studied the Heartless. Donald and Goofy are approached by Leon’s ally, Aerith, who also tells them what they may be looking for. Soon after, Sora meets Donald and Goofy rather hastily in the Third District of Traverse Town, and the three work together to take down a large armored Heartless, the Guard Armor, leader of the Heartless attacking Traverse Town. From there, they decide to travel together: Donald and Goofy to find Mickey, and Sora to find Kairi and Riku.
The three travel to different worlds based on Disney films, finding that the Keyblade also locks “Keyholes”: passages that the Heartless would use to try and take the heart of the world, destroying it and casting it into the Realm of Darkness alongside anyone who lived in the 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 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. 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, leaving Riku into being bested in combat against his former friend. Shamed with losing the Keyblade and being defeated by Sora, Riku flees and meets the same cloaked man Sora encountered back on Destiny Islands, who goads him to give into the darkness.
Sora, Donald, and Goofy, meanwhile, engage and destroy Maleficent after defeating her in battle in the Castle Chapel and then fighting her in her dragon form. 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 in the Great Hall, where the other Princesses of Heart have been held this whole time; 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, though it destroys Ansem’s Keyblade and returns the hearts of the other Princesses to their bodies. 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 and close to the Realm of Darkness itself. Upon being found and finally beaten, despite the various means he tries to fight Sora with, including the World of Chaos, 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, who is 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 her before separating they will all reunite someday. 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.