Riku arrives in what appears to be the Mysterious Tower, but in actuality it is another world called the Symphony of Sorcery. He finds the tower is flooded, so he heads to the top, where he finds Mickey asleep and wearing Yen Sid's sorcerer's hat. Mickey communicates with him telepathically. He summons a musical score and asks Riku to find an item inside called a "Sound Idea" that can dispel the darkness. Riku agrees to help and ventures a long way until he finds the Sound Idea in a snowy area.
After obtaining the Sound Idea, Riku suddenly finds himself in a dark place with a volcano in it. The Mysterious Figure appears and tells him that since Riku learned how to control his own inner darkness, he is no longer of any use to him. As the boy vanishes, the volcano erupts and the fearsome demon Chernabog appears. Riku is forced to battle the fiend, but he emerges victorious. Upon returning, he gives the Sound Idea to Mickey. It joins together with another Sound Idea found by Sora, and together they wake Mickey up. After speaking with him a bit, Riku unlocks the final Sleeping Keyhole, which should put an end to his Mark of Mastery exam.
Back in the real Mysterious Tower, Yen Sid is musing what Lea has been up to: Merlin and the Good Fairies sent him to train in a time vacuum in the hopes that he can learn to wield a Keyblade. They begin to wonder whether or not Xehanort could have followed Sora and Riku into the Sleeping Worlds. Since a version of oneself has to exist at the destination of time travel, that would mean that an incarnation of Xehanort would have to have been at Destiny Islands on the night of the storm for him to follow them. What's more, Xehanort would have to have given up his body for time travel to work. Mickey suddenly remembers that Ansem was without a body prior to possessing Riku, meaning Xehanort could potentially have followed him and Sora into the dreams. Mickey figures that Sora and Riku will return to the real world soon if everything goes according to Xehanort's plan, so he goes to wear he thinks Xehanort will show up. Donald and Goofy want to accompany their king, but Mickey tells them to stay behind. Yen Sid tells Mickey that he sensed Riku in the Realm Between, which narrows down his potential location. With that, Mickey goes to find Sora and Riku as their Mark of Mastery comes to an unexpected end...
