Reverse/Rebirth begins shortly after Riku helped close the Door to Darkness. Having fallen unconscious afterwards, Riku soon wakes up and finds himself in a gray void. A voice calls out to him, telling him he should stay asleep in the void between light and darkness. It explains that he fell asleep because he was unable to overcome the darkness. The voice again encourages Riku to remain asleep in the void, but it tells him that if he truly wishes to leave, he can do so. It manifests a card before Riku and tells him that it will lead him to the truth, but that the truth will be painful. Riku decides to take the card and leave the void. The voice compliments his wise choice, and the card begins to emit a bright light. After the light fades, Riku finds himself in Castle Oblivion, in a hallway identical to the many ones Sora, Donald, and Goofy found themselves in. Riku receives his first World Card, and it leads him to Hollow Bastion.
Upon reaching Hollow Bastion, the disembodied voice returns and tells him that it is just an illusion based on his memories from the time he stayed there with Maleficent. The voice tells him that he would normally meet people he knows from his memories but says nothing more, much to Riku's annoyance.
Riku ventures through the castle, fighting Heartless along the way, until he finds the bedroom he lived in while working for Maleficent. The voice returns and taunts him over abandoning his friends and family so he could gain the power of darkness. Riku gets fed up with the voice and leaves the room.
Riku reaches the foyer, which is also empty. He asks the voice once more where everyone is. It responds by telling him that, since he cast away his relationships to serve the darkness, his heart, and by extension his memories, has become empty, which is why the illusion has no one in it.
Riku continues onward until he reaches the chapel. To his surprise, he finds someone there, although not someone he wishes to see: Maleficent. She tells him that because his heart is corrupted by darkness, the only people he can meet in his memories are those who also serve darkness—in other words, the only people he can meet in his memories are villains. Maleficent says Riku should be grateful that he has darkness in his heart, otherwise his memories would be completely empty. He rebuffs her, but Maleficent urges him to accept the darkness again. Riku declares that if he can only meet those corrupted by darkness in his memories, he'll eliminate them all. She says that if he plans to destroy everyone corrupted by darkness, then he'll have to destroy himself as well. Maleficent transforms into a dragon and attacks, but Riku destroys her. Afterwards, he finds himself back in Castle Oblivion.
In another Castle Oblivion Hallway, Riku is stopped when he hears the voice again. It tells him to surrender to the darkness before finally appearing before him. The voice turns out to belong to none other than Ansem. He tells Riku that he wishes for him to embrace the darkness so he can possess him once again. Riku goes on the offensive, but Ansem effortlessly beats him back. Before he can hurt Riku any further, a glowing ball of light appears. It turns out to have Mickey's voice. The ball flies into Riku, and Mickey tells him that he can always find the light, no matter how deep he falls into darkness.
Riku and Ansem have a brief battle, but Ansem quickly calls it it off. He produces another World Card and tells Riku that he will give into the darkness if he explores his memories some more. Riku accepts the challenge and says that he'll win if he can make it through his memories without falling to darkness. Ansem also empowers the darkness inside Riku's heart, giving him more incentive to use it. He says that he can't force Riku to rely on darkness, but making it stronger gives him one more reason to utilize its power. Ansem says that he'll wait for Riku's decision before vanishing.
Meanwhile, a few Organization members are having a meeting. One of them is Vexen, but the other two, Lexaeus and Zexion, are introduced for the first time. Zexion informs the other two that he has detected two presences in Castle Oblivion's basement. The first was a copy of Maleficent, which has already been destroyed. The second presence is someone who possesses a very similar aura to the Organization's leader, whom they refer to as the Superior. Lexaeus says that they should wait before making any hasty moves.
Back in the basement, Riku notices the presence of darkness covering him, which is so strong that it was absorbed into his skin. Mickey uses his power to send a projection of himself into the room. He tells Riku to never lose sight of the light within the darkness, and he says that he's trying to reach him so they can reunite in person.
From Basements 11-8, Riku can choose to visit Traverse Town, Agrabah, Monstro, and Neverland in any order. This recap assumes he's visiting them in the order he visits them in Kingdom Hearts.
The next world Riku travels to is Traverse Town. Since there were no villains in the world aside from the Heartless, he doesn't encounter anyone else. In this world he fights and destroys the Guard Armor. Meanwhile, Zexion has managed to identify the aura as Riku. Vexen comes to the conclusion that Riku was able to escape the Realm of Darkness with the dark powers he possessed. He further explains that the reason Riku emerged in Castle Oblivion because of his heart's connection to Sora. Zexion is upset that Marluxia has already begun to enact the plan without even telling him and Lexaeus that Sora was in the castle. Vexen calls Marluxia a fool for being so obsessed with Sora, when Riku is the truly interesting one due to his ability to overcome darkness.
Riku's next destination is Agrabah, where he fights Jafar in his genie form. After returning to Castle Oblivion proper, he is confronted by Vexen. When Riku asks him if he's working for Ansem, Vexen tells him that the person he works for is actually Ansem's Nobody who exists in the twilight between light and darkness. The two of them fight, but Vexen then reveals that he was only fighting to collect data from Riku before disappearing.
Riku makes his way to Monstro, which like Traverse Town, has no villains. He fights the Parasite Cage Heartless and leaves.Back upstairs, Zexion and Lexaeus are ruminating on the other members. Zexion is concerned about Marluxia and Larxene's actions, while Lexaeus is more worried about the wild card Axel. Zexion hopes that Vexen's replica will solve the issue.
After traveling through Neverland and defeating Captain Hook, Riku meets the replica of him that Vexen made. He insists that he's better than the original because, unlike the real Riku, he isn't afraid of using darkness. They battle, and the real Riku wins, but the replica uses darkness to push him back. After mocking him for being afraid to use darkness, the Riku Replica runs off. He returns to Vexen, who asks him to go test his mettle against Sora. The Riku Replica insists that he's far superior to both Sora and his real self before heading off.
While looking for the replica, Riku encounters Ansem again. He tells Riku that the replica should serve as a model of what he should try to be, since he doesn't fear darkness. Riku again tries to attack, but Ansem repels his strike again before giving him new World Cards.
From Basements 7-4, Riku can choose to visit Atlantica, Halloween Town, Olympus Coliseum, and Wonderland in any order.
Riku enters Atlantica and beats the gigantic, trident-wielding form of Ursula. Meanwhile, Vexen is going to meet Axel and Larxene. The scene plays out as it did in Sora's story: Vexen summons the Riku Replica to test Sora; this time, though, more of the scene is shown. Vexen looks at the card Axel gave him, realizing it is a card of Destiny Islands. Larxene tells the Riku Replica that with Naminé's powers, they can give him the real Riku's memories so he can be an exact copy. The replica isn't thrilled with this idea, viewing the real Riku as a coward who fears darkness. None of the Organization members care about the replica's opinion, planning to use the Destiny Islands card to give him the memories despite his protests. The Riku Replica, seeing his creator turning against him, brandishes his Soul Eater and tries to attack them, but Larxene blasts him back with ease. He screams as Larxene approaches him and inserts the memories.
While Riku is traveling through Halloween Town to defeat Oogie Boogie, Larxene is taunting Naminé about being forced to mess with Sora's memories, which clearly upsets her. The Riku Replica returns, now fully convinced that he is the real Riku. He repeats his fake promise to Naminé and shows her the false good luck charm before leaving. Larxene continues to belittle Naminé, but she insists that Sora will never truly forget Kairi, especially since Naminé herself is actually Kairi's shadow. Larxene tells her that since she was created from Kairi, that should just make it easier for her to insert herself into Sora's memories, giving her the opportunity to be a real person, at least to Sora.
Riku heads for Wonderland. He doesn't encounter the Queen of Hearts or her Card Soliders; he only fights Heartless in this world, including the giant Trickmaster. While he does that, Lexaeus goes to report to Zexion that Vexen has been killed by Axel, much to Zexion's disgust. Lexaeus fears Sora, but they note that they cannot eliminate him, because the Superior needs Sora to accomplish his goal. Since Sora has fallen under Naminé's control, they decide that they should combat his light with Riku's darkness. Meanwhile, Riku only has one World Card left. He wonders if conquering this illusion will free him from the darkness.
The illusion leads Riku to Olympus Coliseum, where he fights and defeats Hades. After leaving the world in the card, he meets Lexaeus, who also tries to tempt him into using darkness. When Riku refuses, Lexaeus fights him. The ensuing duel is intense, and both combatants wear themselves out. Riku tries to finish Lexaeus off, but he smacks Riku into the ceiling. After Riku collapses onto the floor, Lexaeus begins to approach him, but a dark aura envelopes him and transforms him into his Dark Mode. "Riku" stands back up and finishes Lexaeus with a single blow. He speaks with the same dual voice he had in Hollow Bastion, signalling that Ansem has regained control of Riku. As he fades away, Lexaeus realizes that the aura he sensed before, the aura that was so similar to the Superior, actually belongs to Ansem, who was lurking inside Riku's heart the whole time. Lexaeus apologizes to Zexion for overestimating himself before being destroyed.
Riku finds himself trapped within a void of darkness within his own heart. He calls out to Lexaeus, but only the voice of Ansem responds. As Riku begins to recall the power of Ansem, he manifests in front of him. Ansem approaches Riku and nearly overcomes him, but Mickey's ball of light, which he inserted into Riku's heart earlier, reappears and urges him to resist. Mickey appears as a projection once more, angering Ansem. With Mickey's help, Riku is able to regain control of his body.
Riku wakes back up in Castle Oblivion. He calls out to Mickey, but there is no response. He realizes that even though the king isn't there himself, he is still connected to his heart. Meanwhile, while Zexion is recounting the Organization members who have died, Axel shows up. When Zexion tells him that he thinks he will be the next to die, Axel reveals that he survived his second battle with Sora. Since Sora thinks that he's dead, he won't pose a threat to Axel anymore. Axel says he bets Marluxia will be the next to die due to his betrayal. Zexion informs Axel that his original plan was to use Riku to destroy the traitors, but since most of them are already dead, that is no longer necessary. Axel tells Zexion that Riku is nothing more than a threat now, so he should destroy him. Zexion says that he prefers not to get his hands dirty, but he asks Axel if he managed to obtain a card of Destiny Islands. Zexion goes to confront Riku in the hopes of eliminating him, all while Ansem is still vying for control of his heart...
