From Floors 7 to 10, Sora can choose to visit Atlantica, Neverland, the 100 Acre Wood, and Hollow Bastion in any order. This is assuming he's visiting them in the order he does in the first game.
Sora and company's next stop is Atlantica, but since this version is an illusion, they retain their normal appearances and can breathe underwater just fine. As they ponder how they can do this, they are approached by Ariel holding her father's trident. She tells them to pretend like they didn't see her before swimming away. They decide to investigate, ending up at Triton's palace.
At the palace, they find Sebastian, who is worrying about the loss of the king's trident and the disappearance of Ariel. They decide not to tell him what happened since it would make him lose it, so they go to find her themselves.
The trio finds Ariel with the trident in her grotto, when suddenly Ursula appears. It turns out that Ariel's friend Flounder has gone missing, and Ursula offered to save him in exchange for Ariel letting her borrow the trident for a little bit. Ariel hesitates to hand over the trident. Ursula says that Ariel can take her time deciding, but while she does, Flounder will continue to be in trouble. After the sea witch disappears, Sora, Donald, and Goofy confront her. They tell her not to trust Ursula, but Ariel says that she is the only person who knows where Flounder is. Sora offers to accompany Ariel to meet her, saying he knows what it's like to have a friend in need.
They make it to Ursula's cave, where Ariel demands proof that the sea witch will uphold her end of the bargain before she hands over the trident. Ursula agrees and summons Flounder, revealing that he was actually in her clutches the whole time. Ariel gives it to Ursula, who promptly declares herself the ruler of the seas. She holds up her end of the deal and releases Flounder, but then she proceeds to say that she'll give Ariel a "one-way trip" to another world. Ursula grows to gigantic proportions and battles Sora, who succeeds in destroying her.
After the battle, Flounder apologizes and explains that he was tricked by Ursula. Ariel wants to return the trident, but Sora points out that she'll get in trouble if Triton finds out that she was the one who stole it. They suddenly hears the voice of Sebastian, who's looking for Ariel. Sora suggests that Ariel lies and says that Ursula stole it and that she took it back from her, but Ariel decides against it, deciding to do the right thing and tell the truth. Jiminy chides Sora for suggesting that Ariel lie, and they leave Atlantica to return to Castle Oblivion.
As soon as he emerges from Atlantica, Sora is shocked to see none other than Riku, the very person he had been trying to find all this time. Sora asks Riku why he's in Castle Oblivion, but Riku seems to take it as an insult, stating that Sora must not really be looking for him but someone more important. Sora tries to retort, but Riku says that Sora had probably forgotten about him. Sora tells Riku that he was the reason he entered the castle in the first place, but Riku states that he'd given up on him, and that he only cares about finding Naminé now. Riku says that Sora never cared about Naminé's feelings and that she doesn't even want to see him again. When Sora asks why, Riku tells him to search his memories, implying it was Sora's fault she left. Riku transforms into his Dark Mode, wearing the same outfit he had when Ansem possessed him, and tells Sora that he won't let anyone near Naminé. They battle, and Sora wins, but Riku runs away.
Sora tries to chase after him, but by the time he reaches the next floor, Riku is already gone. Jiminy ponders if Ansem could be controlling him again, but Sora says that would be impossible, since they seemingly destroyed him. As Goofy wonders what happened to Riku, Donald realizes that since he was with Mickey, the king could be in danger if Riku has again fallen to darkness. Jiminy tries to cheer Sora up by stating that Riku must have gone crazy due to Castle Oblivion messing with his memories, meaning that if they can find out why their memories are being erased, they may be able to get the old Riku back. Sora is still despondent, but Donald and Goofy cheer him up by reminding him of their rule: "always smile!" Sora feels a bit better, and they press onward to find Naminé and Riku...
