In a flashback, Braig walks into Ansem the Wise's study and finds Even and Ienzo clouded in darkness. He demands Terra-Xehanort to explain what is going on, but Xehanort brandishes his Keyblade, causing Braig to wonder if Xehanort had either regained his memories or never lost it. However, Xehanort simply stabs Braig, declaring that his name is "Ansem."
Fast-forward over a decade later, and Sora and Riku have been sent to Yen Sid's study for the Mark of Mastery. For this test, both Keyblade wielders are to venture into the Realm of Sleep and seal seven Sleeping Keyholes to gain the power of waking, a skill necessary to save the missing Keyblade wielders Terra, Aqua, and Ventus. Even though both are sufficient Keyblade wielders on their own, they lack formal training, so they are to begin anew with a clean slate. To access the Sleeping Worlds, which fell during the Heartless invasion, they need to return to the Destiny Islands from just before it fell to darkness.
Sora and Riku return to the Destiny Islands as their younger selves. As Sora muses on how far a raft could take them, Riku expresses optimism for the future.
As they set sail, they discover that a storm is heading their way, which soon destroys their raft. They encounter a phantom of Ursula, whom Sora had faced many times before in the past and is now seeking revenge on the orders of someone. But there is no time to find out who this someone is. The young Keyblade wielders face off against the phantom and ultimately slay her again, opening up a Sleeping Keyhole. But little do they know that someone is watching them, musing to himself that this world has been connected...