On the next floor of Castle Oblivion, Riku is suddenly stopped by a huge earthquake. He notes that a very strong presence just faded away. Zexion appears and informs him that Marluxia, the lord of the castle, has just been destroyed by Sora. Riku is excited to hear that his best friend is also in the castle. Zexion tells Riku that he can help him reunite with Sora, but that he may want to think twice before doing so. He says that since Riku still has the darkness of Ansem within him, and since Sora's fate is to fight the darkness, that means that Riku is still his enemy. He hands Riku the Destiny Islands card, telling him to find out the truth for himself.
Finding himself back home, Riku is surprised to realized that he's actually glad to be back, considering that his desire to leave the islands is what caused everything since then. Much to his surprise, he sees Tidus, Wakka, and Selphie in the distance. He runs over and tries to talk to them, but they simply vanish.
Riku proceeds through the island until he finds Kairi. Just like the others, she simply disappears. Zexion shows up and reminds him that he can only meet beings of darkness, like the Disney villains, inside the memory worlds, because he forgot his friends. Riku insists that he didn't forget, but Zexion tells him that he was the one who destroyed the world. They find themselves on the floating island on the night of the storm, where a memory version of Riku appears. It is revealed that Riku was the one who opened the world's Keyhole, allowing the Heartless to invade and ultimately destroy it. Zexion tells Riku that on that day, he gave himself to the darkness. He tells Riku to look at what he really is, and the memory Riku transforms into the Darkside Heartless.
Riku destroys the Darkside, but then he sees Sora standing at the edge of the island. A door appears, giving him the option to go back, but Riku decides to confront his friend. He tries to talk, but Sora turns around and attacks him. As they lock blades, Sora tells Riku that he sees what he has become. Sora blasts him down with a beam of light from his Keyblade, and he says that if light can hurt him, then he must be a servant of darkness. Sora charges up a huge beam of light, blinding Riku.
Riku finds himself falling into a void of light. He begins to fade away, but a vision of Kairi appears and tells him that neither light nor darkness can destroy him. She tells him that he should embrace the powers of both light and darkness to make himself stronger. Kairi says that as long as he doesn't become consumed by it, Riku can command the darkness along with light. Riku asks himself if he can face his friends. Naminé briefly appears alongside Kairi, both girls asking him if he wants to run away. Riku says that he doesn't want to run. Naminé and Kairi disappear, and Riku resolves to use the powers of both light and darkness.
Riku uses the power of darkness to break free from the void of light. Finding himself back on the floating island, the "Sora" who attacked him actually turns out to be Zexion in disguise. Riku says that no matter how much light he cloaked himself with, Zexion couldn't hide his darkness from him. With his illusion destroyed, Zexion has no choice left but to fight. After being defeated, he wonders Riku how he suddenly gained so much courage. Riku charges at him and attacks, but Zexion teleports away before he can be destroyed.
While Riku finds his way out of the illusion of Destiny Islands, an injured Zexion returns to the castle, asking himself how the boy can use the darkness in such a way. He turns around to see Axel and "Riku." Zexion is horrified at first, but he calms down after realizing it's just the replica. He tells Axel that they can defeat the real Riku with the replica, but Axel ignores him and speaks to the replica, asking him if he wants to become real. He tells the Riku Replica that if he gains more power, he can become a real person, no longer a mere clone of Riku. Axel tells the replica to take Zexion's power. He protests, but the Riku Replica grabs him and begins to drain his power. Axel looks on with glee as Zexion fades away. With that, all the members of the Organization assigned to Castle Oblivion are dead, with the exception of Axel.
Back in Castle Oblivion, Riku hears Ansem's voice calling out to him. He says that, since Riku has chosen to embrace the darkness, soon it will consume him. Ansem holds Riku in a T-pose as he tells him that he has nearly regained control of his body. Suddenly, another ball of light from Mickey appears and flies into Riku's heart, keeping Ansem at bay for a little longer. Mickey himself then appears. Riku realizes that this is the real Mickey and not just a projection. The king explains that, just like Riku himself, he got a card that helped him escape the Realm of Darkness. Mickey says that the card led him to Riku, meaning that it must have wanted him to have it. The king hands Riku the card, which leads him to Twilight Town.
Just like Sora, Riku has never been to Twilight Town. He looks around, but Mickey is nowhere to be seen. Ansem appears and tells Riku that they must fight one-on-one. He raises his Soul Eater to fight, but then he lowers his weapon, saying that he doesn't have the darkness that Ansem does. Riku realizes that this Ansem disguised himself to guide Riku out of the void. He transforms and reveals himself to be a man with a face covered in bandages. The man introduces himself as DiZ. He tells Riku that he must first meet Naminé and then make a choice. Riku doesn't yet know who Naminé is, but DiZ assures him that he will learn soon before disappearing.
Riku makes his way to the old mansion, where he is confronted by his replica once again. He notes that the real Riku has changed, and he expresses his desire to become a real person. He says that even with his new powers, he still can't truly be his own person as long as the real Riku exists. The Riku Replica attacks, and they battle again. The real Riku comes out on top again, and the replica begins to fade away into darkness. As he does, he asks what happens to a replica when it dies. As he fades away, he finally accepts himself for who he is.
After destroying the replica, Riku makes his way out of Twilight Town. Back in Castle Oblivion, he finds Naminé in the room with Sora's pod. He recognizes her from the vision of Kairi he had before his fight with Zexion. Naminé explains to Riku why Sora is sleeping inside of a pod before telling him that he must choose whether or not he wants her to seal away the darkness within his heart. If he accepts, Ansem will no longer be able to torment him, but he will lose all his memories since the moment he embraced darkness for the first time. Riku declines Naminé's offer and decides to take on Ansem himself. He tells her that if the darkness threatens to overtake him, he will simply use its power for himself to lead him back. Naminé reveals that she was actually hoping Riku would decline her offer. He tells her that he knew that the "Kairi" he met in Destiny Islands was actually her all along, because the two of them have the same aura.
Riku proceeds to the next floor of the castle, where he reunites with Mickey and DiZ. Riku asks Mickey if he knows who DiZ is. The king says that he feels like he's met him before, but he doesn't know for sure what his true identity is. When Riku asks, DiZ just gives him a cryptic answer that means nothing. He gives Riku and Mickey the black cloaks that the Organization uses, telling them that it will conceal their identity from the Nobodies. He says that the reason the Organization itself wears them is because even they can't control darkness. When Riku says that he still wants to face Ansem, DiZ gives him a final card with an image of Castle Oblivion itself on it. He explains that the card will cause the darkness of Ansem within him to manifest itself.
Before using the card, Mickey offers to help Riku, but he insists on doing it alone. He asks Mickey to destroy him if Ansem ends up possessing him again, but Mickey refuses, saying he knows Riku will win. With that, Riku holds out the card and proceeds through the door. He finds himself inside a hallway, where Ansem says he'll be waiting for him at the end of the labyrinth of darkness.
Riku finally makes it to the end of the castle, where Ansem appears. He tries one last time to tempt Riku into giving in, but when he refuses, Ansem lets out a huge burst of darkness. Riku finds himself in an illusion of the dark abyss where Sora defeated Ansem at the end of his previous journey. Ansem summons his Guardian, and the two of them fight. The battle is long and fierce, and the two combatants eventually find themselves back in Castle Oblivion. Ansem launches an energy ball at Riku, but he manages to block it. Riku runs at him and slashes straight through him with the Soul Eater. He has seemingly beat Ansem for good, but he warns Riku that it is far from over. He says that as long as darkness exists within Riku, he can and will return. Ansem explodes into a huge burst of darkness that consumes him. In the black void, Mickey shows up and helps him find his way out of it.
Riku and Mickey finally reach the exit of Castle Oblivion. Riku says that he can't return home yet, because he can still feel Ansem's presence within him, no matter how weak it may be. Mickey tells him that he is the only one he's ever known who uses both darkness and light, and that meeting Riku has convinced him that darkness isn't inherently evil. Riku calls him "Your Majesty," but he says that he and Riku are now friends, so he should just call him Mickey.
Riku and Mickey, now wearing leave Castle Oblivion and find themselves at the grassy field where Sora, Donald, and Goofy arrived after closing the Door to Darkness. They reach a crossroads where they find DiZ waiting for them. He asks Riku to choose either the road of light or darkness, but Riku refuses to pick either, choosing the middle road instead. When DiZ asks him if that is the road of twilight, he says "No. It's the road to dawn."
Afterwards, Riku and Mickey are shown walking towards Hollow Bastion. Riku feels a pain in his heart and falls down, but when Mickey tries to help him, he gets back up and dismisses it. Meanwhile, in Twilight Town, a boy the same age as Sora is walking past a group of three friends, when he is approached by Axel of the Organization. The boy and Axel head to the town's clock tower and eat ice cream together. In a flash-forward scene, DiZ and Naminé, who are watching over Sora's pod, are approached by an older Riku. The mysterious boy at Twilight Town is shown meeting the group of friends he walked by earlier. He climbs to the top of the clock tower again, staring into the sky, and there is another poem:
"Beyond the path without you
is a forgotten promise to keep.
We may have walked side by side,
but now we go on back to back.
And though our paths may not cross,
all paths are connected to somewhere.
When I arrive at where you are,
we may not appear to be as we were...
But we'll make another promise to keep."
