Sora and company eventually make it into Hollow Bastion, the lair of Maleficent and her group of villains. Sora realizes there is something familiar about the castle, but Donald brushes it off.
As the trio make their way up towards a rock near the castle, they encounter the Beast, who has been pursuing Princess of Heart Belle. However, Riku arrives and incapacitates the Beast before revealing that he, not Sora, is the Keyblade's real chosen wielder. Despite Sora's protests, Riku points out that Sora was merely the delivery boy and steals the Keyblade for his own; Donald and Goofy have no choice but to follow Riku since he has the Keyblade. Before leaving, Riku tosses Sora his old wooden sword.
But Sora isn't done yet, and teams up with the Beast to chase Riku. With the Beast's help in defeating the Heartless, Sora manages to break into Hollow Bastion's basement and opens up the chapel doors. However, the Beast is distracted by a Heartless copy of Belle. Riku arrives and prepares to kill Sora, but Goofy blocks the attack; despite the King’s orders, Sora is still a dear friend. Donald follows suit and rejoins Sora’s side, prompting Sora to realize that he doesn’t need the Keyblade; he has his heart.
With those words, Sora regains the Keyblade and proceeds to defeat Riku, who then retreats. As Riku mopes about his defeat, he encounters the robed figure, who offers more power and promptly possesses him. As Maleficent assembles the Princesses of Heart to reveal the Final Keyhole, "Riku" approaches, now wielding a dark Keyblade...
With the Beast on their side, Sora and company fight their way up Hollow Bastion. They make it into the castle chapel, where Maleficent meets them. She engages the heroes in an epic battle, but they defeat her, forcing her to retreat with them in pursuit. But "Riku" arrives and reveals that his Keyblade can unlock people's hearts and demonstrates it by stabbing Maleficent, transforming her into a dragon, but they overpower her, too.
As the Beast leaves, Sora and company chase "Riku" into the sanctum of the castle, where Sora finally reunites with an unconscious Kairi. "Riku" explains that she has lost her heart, but reveals that she's a Princess of Heart and that her heart is inside Sora. At this point, Sora is so confused that he outright knows that this isn't Riku and demands to know who he is:
Ansem prepares to rip Kairi's heart from Sora to complete the Keyhole, but Sora retaliates and duels Ansem in a lengthy battle. Though Sora seemingly sends Ansem packing, the Keyhole is about to explode, so Sora stabs himself with Ansem's Keyblade, freeing all seven princesses, even Kairi, but at the cost of his own heart. No sooner than Sora disappears in her arms does Ansem reappear, now taking on a different form but still using Riku’s body. He declares that Kairi is now useless now that the Keyhole is complete and starts to approach her, but Donald and Goofy step up to defend her. Suddenly, Ansem finds himself unable to move, and a transparent Riku appears, who’s attempting to hold off Ansem in his body as long as he can. He warns Kairi, Donald and Goofy that a massive army of Heartless are coming, and that need to leave Hollow Bastion immediately. As Shadows start to appear around them, the trio make a break for it.
Shortly after, Sora reappears, now a Heartless himself, and takes pursuit and catches up with them at the bottom of the castle. Donald mistakes Sora for the enemy, but Kairi senses Sora's true self, and she ends up shielding him from the rest of the Heartless surrounding them— an act that knocks them back with a bright flash of light and restores Sora to normal. Together, with Kairi saved, Sora and company leave for Traverse Town again...
Tropes:
- And Then John Was a Zombie: Sora is transformed into a Heartless. He gets better, though.
- Demonic Possession: Riku ends up possessed by Ansem after the former loses the Keyblade to Sora and decides to open his heart further to the darkness.
- Disc-One Final Boss: Maleficent is fought and defeated by Sora's party before it's revealed that she's not the mastermind behind the Heartless after all.
- Heroic Sacrifice: Sora stabbing himself with Ansem's Keyblade. He gets better.
- Nemesis Weapon: Ansem's Keyblade, in contrast to Sora's. While they both look similar, Ansem's has a more sinister color scheme and looks incomplete.
- One-Winged Angel: Maleficent's dragon form, courtesy of Ansem.
- The Power of Friendship: What Sora realizes is his inner strength, and what allows him to regain the Keyblade.
- The Unchosen One: As it turns out, Sora isn't the Keyblade's real chosen one; it's Riku. Although Sora regains the weapon thanks to his bonds.
- Wham Line: "It is I, Ansem, the Seeker of Darkness."
