Sora is surprised to find himself finally back home in Destiny Islands. The first thing that comes to mind is his memory of playing with Naminé when he was younger. He suddenly encounters his old friends Tidus, Wakka, and Selphie, but he can't seem to recall their names until they say them themselves. They assume that Sora was distracted because he was thinking about a certain girl. They decide to leave so he can be alone with the girl.
Sora makes his way to the small island where he used to play with Riku. Sure enough, he finds his best friend there. It takes him a moment, but Sora manages to recall his name. He asks Riku if he's still under Vexen's control, but he doesn't understand. Since this is a world crafted from his memories, this illusionary Riku acts like Sora remembers him before he fell to darkness. Just like the others, Riku makes mention of a specific girl. Sora tries to ask him about her, but they are interrupted by an earthquake. Riku tells Sora he needs to protect the girl and leaves.
As Sora searches for the girl, a storm brews, and a big ball of darkness appears, just like the night when the Heartless destroyed the islands. Sora finds himself on a floating island in the sky, where he meets the Heartless Darkside once more. It is no match for Sora and is easily destroyed.
Sora resumes his search for the girl, whom he assumes to be Naminé. He turns around and sees her standing behind him. Sora is elated to finally meet her after all he went through, but even though she's happy to see him, she can't help but feel guilty for messing things up. She explains that she was extremely lonely, so she called Sora to Castle Oblivion. Sora says he just cares about keeping his promise to her, but Naminé says she isn't supposed to be there. Another Naminé, this one being transparent, appears behind him and says that the solid Naminé isn't really her. The Naminés ask Sora if she was really the one he was looking for. Sora says she was, and he pulls out the good luck charm as proof. The solid Naminé is happy to see it, but the transparent one tells him not to believe her. She tells Sora to search deep within his memories to find who is really the most special person to him. Sora begins to answer with Naminé's name, but suddenly the good luck charm flashes and returns to its true form. He looks at the solid Naminé, who then turns into Kairi.
Sora finds himself back on Destiny Islands before it was destroyed. He can't remember who Kairi is, but he feels that he knows her. He calls out Naminé's name, but she's nowhere to be found. With nothing left in Destiny Islands for him, Sora moves on.
Sora makes it to the next floor of Castle Oblivion, where he finds Naminé waiting for him. She confirms to him that she isn't really the most special person to him. He asks her why the other girl (Kairi) is so special to him if he can't even remember her name. Before Naminé can explain, Riku reappears. He tells Sora that he's the one who needs to protect Naminé, not Sora. The two of them fight again, and Sora seemingly wins. Sora lets his guard down as he approaches the weakened Riku, but he suddenly blasts Sora with Dark Firaga and knocks him down. As Riku prepares to finish Sora, Naminé begs him to stop. It doesn't work the first time, but the second time, Riku's forehead flashes again as he collapses to the ground. To Sora's horror, Riku isn't moving at all, not even breathing.
Larxene appears and finally reveals the truth to Sora: the "Riku" he had been fighting the entire time in Castle Oblivion wasn't actually the real Riku, but merely a replica created by Vexen. She explains that the fake Riku's memories were artificially implanted by Naminé herself. Sora learns the truth about Naminé as well: she has the power to alter the memories of anybody connected to Sora, including Sora himself, and that's why he had been losing his memories. Meanwhile, his memories of Naminé were fake. The Organization put these fake memories into Sora's heart in an attempt to manipulate him into doing their bidding. Larxene also reveals that Axel was the one who let Naminé escape, ruining their plans as part of his efforts as a double agent sent to eliminate the real traitors. As Larxene prepares to fight Sora, Naminé attempts to stop her, but the Savage Nymph smacks her down, blaming her for foiling their plan. Sora calls out to Naminé. Larxene asks him why he still cares for her even though he knows she manipulated him, but Sora responds by saying the promise is real to him, even if the memory of it is fake.
Larxene gets extremely annoyed that Sora would cling to a memory he knows to be fake. She prepares to destroy him, but Donald and Goofy come to the rescue, Goofy throwing his shield at her and Donald using Curaga. The three of them do battle with Larxene and finally manage to destroy her. She fades away, unable to comprehend the fact that she could lose. After the battle, Donald and Goofy introduce themselves to Naminé. She then explains how she changed Sora's memories. She changed it little by little, in bits and pieces. She confirms that the promise Sora made was fake as well, and that she never knew Sora prior to him setting foot in Castle Oblivion. Naminé tells them that she can fix their memories if they reach the 13th floor, but they'll have to deal with Marluxia when they get there. She also confirms that he was the one who forced her to mess with their memories, although she also admits part of her wanted to do it so she could make someone care about her.
Sora tells Naminé not to cry. She says she doesn't deserve to, but Sora says that's not why he doesn't want her to. The real reason he doesn't want her to cry is because he still cares about her. Even though the memories are fake, he says he'll still feel guilty if she cries. Naminé is blown away by Sora's kindness, but Donald and Goofy tease him by saying he always acts like that around girls. They tell her to stay behind and watch the Riku Replica so she can stay safe. With that, Sora, Donald, and Goofy ascend to the next floor, heading for the top of the castle to take on Marluxia...
