A year has past since the second Keyblade War. However, a old evil, sealed away for a thousand years, begins to stir anew. Its awakening spelling doom to all the worlds. Warned by Naga, Yen Sid summons the Guardians for this old threat, to finish what he helped to end.
Aqua thought she would be stuck in the Realm of Darkness for eternity, but when she comes across a being of Darkness-that-isn't-quite-Darkness, did she simply trade one nightmare for another?