Summary
By bringing about the defeat of both Ansem, Seeker of Darkness and Xemnas, Sora and Riku have unknowingly brought about the return of Master Xehanort, the man who had destroyed the lives of three Keyblade apprentices and their Master over a decade ago and have since been wreaking havoc throughout the worlds in his various forms. As soon as he realized this, the wise sorcerer Yen Sid immediately summoned the two young wielders to his tower and requested they undergo the Mark of Mastery exam in order to combat the threat that approaches. Although they have overcome many obstacles and grown in such a short period of time, they do not yet possess the powers they require to protect the worlds and must dive into the Sleeping Worlds to start over with a blank slate.
The two of them were sent back to the point in time when their adventure was just about to begin: The Destiny Islands right before the storm that had changed their lives forever. Sora and Riku departed their home on the tiny raft they had built with their own two hands and set sail towards the horizon, not knowing where they'd go and who they'd meet, unbeknownst to them that they were being watched by the brown robed man. They didn't set sail on the exam; they were drifting towards their ultimate demise. They unlocked the first Keyhole, and dove into the Sleeping Worlds.
However, they noticed upon their arrival that they were separated from one another, traveling through different planes of the same world that, although separated, still affect each other. They're dreams; dreams that the sleeping worlds are having at the exact same time. They dream of those who had once resided in that world or still do, recreating them to act upon their given roles, and although they may look familiar, Sora and Riku must not be fooled by their own dreams and memories. Rather than relying on pre-established bonds, they must forge new connections that will lead them out from dream to dream, and with each that they surpass, they'll unlock another Keyhole and release that world from their slumber, allowing it to return to the Realm of Light.
As Sora travelled through the worlds meeting old friends and making new ones, he must confront the familiar faces that plagued his journey in the past; the boy clad in red and black with a familiar face that stares hauntingly at him in a crazed glee, the former leader of the Organization that had been defeated by Sora's own hands, both accompanied by a new silver-haired individual in the black cloak. It seems like whenever the two crossed paths, it only served to dissuade him from his goal, creating doubt within his heart. Although he passionately believes in what he knows as truth, it doesn't help that their cryptic messages strike almost a little too close to home.
On the other hand, Riku dwells on the past he had been struggling to leave behind. He had grown stronger, and atoned for his actions. His friends forgave him for hurting them, and they knew that he had been manipulated. All he wanted was to protect them, no matter the cost. Even if his methods differed greatly and conflicted with personal morals, everything was alright in the end. What's done is done and he should just move on, right? Riku walks the road to dawn. He travels through the worlds, and is challenged, and changed; his path will always remain straight and true.
Sora and Riku come to a point where the end should have been in sight. All seven Keyholes had been sealed, but they were still in the same clothes, in their younger forms. Just what was going on? As if an answer from the depths of darkness, the two are greeted by not only those who've been haunting them throughout their journey, Xemnas and Ansem, Seeker of Darkness, but also those they had believed to have perished long ago, in particular the former Number II of Organization XIII, Xigbar.
This whole thing was a set-up. From the very beginning, they had been guided away from the Mark of Mastery and walked through Xehanort's plans. Xehanort... he had set the entire thing up. They were able to track Sora because of the X that had been marked on his chest, the Recusant's Sigil. The silver-haired man who had been following them from world to world was actually Master Xehanort himself, but from the distant past. He had been gathering his various forms scattered throughout time and space and brought them all there on that day to welcome back the Xehanort from the most distant future. Sora had been completely unaware the entire time, and instead dove into dream after dream, to the point where he had chased them so deep that he became trapped into the deepest abyss, unable to wake. Now that he had succumbed to sleep and allowed the darkness to seep in, he would be perfect as Xehanort's final vessel.
It turned out that Riku had noticed from the very beginning upon seeing the man in the brown robe that trouble was amidst, and instead dove into Sora's dreams to protect him, assuming the role of a Dream Eater. He saw Sora drifting in sleep just as he once did, chasing what wasn't there and unable to distinguish them from reality. He couldn't just leave his best friend in such a state, and fought tooth and nail to get to him. The bonds that had helped Riku get so far, the memories that supported him along the way, of the one who had first encouraged him to reach towards the outside worlds gave him strength that surpassed all others. He finally had the strength to protect what matters, and dove once more to rescue Sora from Xehanort's clutches.
Why did Xehanort want Sora to begin with? The legend of the Keyblade War and the χ-blade that had been destroyed as a result of it were the keys. The χ-blade shattered into a total of 20 pieces: 7 of light and 13 of darkness. the 7 of light were no doubt the princesses and such had 7 guardians of light to correspond to them, but for the 13 of darkness, Xehanort had intended to use Organization XIII; taking empty shells and filling them with the same heart and mind. He hid the knowledge of hearts being able to regrow from the Nobodies who yearned to be whole again in order to turn them all into Xehanort. Ultimately that plan failed, and so brought Xehanort to this moment. His 13 selves would join together and confront the guardians who were still lacking their key members, and usher in a new era.
Once again, his plan was squandered by the heroes of light. Sora was rescued from his ultimate fate as Xehanort's 13th seeker of darkness and new vessel and he had Riku were able to return to the Realm of Light. However, Sora continued to slumber as he fell deeper into the abyss and just couldn't wake up. For the sake of his best friend, Riku dove back into his dreams once more to bring him home, just as Sora had once done for him. With the new powers he had gained from his adventure in the Realm of Sleep, and the bonds he has with his friends, he was ultimately successful, and it was because of his bravery and his accomplishments that named Riku the new Keyblade Master!
Yen Sid wasted no time in sending Riku out on his first mission: to retrieve a special guest, the seventh princess of heart, Kairi. What was it he planned to do with Kairi? They would soon find out.
