With the Final Keyhole sealed, Sora and company decide to pursue Ansem to the End of the World, where the remnants of all the worlds consumed by Heartless are. They realize that if they defeat Ansem, the End of the World might be no more, but decide to press onward. Sora takes one last look at Kairi's lucky charm, and remembers his promise to return it.
Indeed, the End of the World proves to be utterly chaotic. Powerful Heartless lurk around every area, meteors often sending the party into forced fights with them, and they eventually stumble upon a World Terminus where they have to travel back to every world they have ever visited in the past, defeating more Heartless along the way. During this time, Sora discovers a strange machine within the depths of Hollow Bastion encoded with a message, saying that in order to seal the famed "Door to Darkness," it requires two keys- one of light, one of darkness.
Exiting the World Terminus, the trio encounters Chernabog, a famed demon of darkness, and defeat him in combat before descending through his volcano. They pass through more remnants of worlds destroyed in the past, and enter the Final Rest, where they encounter a door Sora had seen in his Dive to the Heart.
Inside the door is the remnants of Destiny Islands, and alongside it is Ansem himself. Ansem states that Riku was so desperate to leave the Islands whatever the cost, even if it was darkness, and assumes his true form once more. He then pontificates on darkness, saying that all worlds and hearts will return to the same darkness whence they came, as darkness is the heart's true essence. But Sora refutes the claim and counters that deep down there is a light that never goes out. Unimpressed, Ansem begins to battle the trio, but they reign victorious, forcing Ansem to leave.
With an opening inside the remnants of the Islands, only Sora is able to make it in, where he fights the same beast he fought during the islands' destruction before fighting Ansem again. Upon defeat, Ansem sends himself and Sora's group into a dark void containing the Door to Darkness, which he believes holds the heart of all worlds: Kingdom Hearts. He fuses into a massive battleship known as the World of Chaos and imprisons Donald and Goofy before engaging Sora in a final showdown.
The ensuing fight is brutal, with Sora first fighting a massive Ansem before raiding the battleship and rescuing Donald and Goofy. The trio destroy the World of Chaos's core before finishing Ansem off. A weakened Ansem opens the Door, pining for Kingdom Hearts to fill him with darkness, but Sora retorts that Kingdom Hearts is light. Indeed, when the Door opens, light floods through, seemingly destroying Ansem. With Ansem gone, the trio hurries to the Door and attempt to close it. A restored Riku appears and tries to help them on the other side, but time is slowly running out; Heartless are seen gathering in the Realm of Darkness.
Just then, a beacon of hope appears: Mickey Mouse himself, wielding his own Keyblade and preparing to seal the door on the other side with Sora. Mickey explains that there will always be a door to the light, and Riku tells Sora to take care of Kairi for him. As Sora brings back many fond memories of their childhood, the door closes for good.
When Kairi appears on the slowly restoring Destiny Islands, Sora runs up to her and tells her that though the worlds are saved, he needs to go and promises to return. With the End of the World destroyed, Destiny Islands comes back. Sometime later, Kairi visits the Secret Place and sees Sora's drawing of them sharing a paopu fruit; shedding a tear, she proceeds to finish the drawing.
Meanwhile, Sora and company (Donald and Goofy having switched to their traditional Disney garb) are walking down a grassy field, unsure of where to find Riku and Mickey. Pluto appears with a new letter from Mickey, prompting Sora to pursue him with Donald and Goofy not far behind.
One year later, a hooded boy wielding two Keyblades and looking for Sora meets a blindfolded Riku in a dark city...
Tropes:
- All the Worlds Are a Stage: The World Terminus allows the party access to all past worlds they've been to (at least, those with a Keyhole; note that Monstro, who doesn't have a Keyhole, is absent). If the Keyhole has been sealed, they can skip that world.
- Attack of the 50-Foot Whatever: The World of Chaos.
- Giant Space Flea Out Of Nowhere: Chernabog's boss fight comes completely out of nowhere.
- Sequel Hook: "Another Side, Another Story" and "Deep Dive" are this to Kingdom Hearts II and, by extension, Kingdom Hearts: 358/2 Days.
- Shoo Out the Clowns: Subverted. Just before the fight with the Darkside, Donald and Goofy are locked out of the battlefield, and remain that way during Sora's third bout with Ansem. Later, the two are also imprisoned during Sora's first fight with the World of Chaos, though he is able to break them out for the final fight.
