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.

Summoned by Master Yen Sid for their Mark of Mastery, Sora and Kairi venture of into a new reality. But what they don't realise, is that this "reality" is anything but, and is a place where Hearts are tormented in their worlds fine hours. Can they survive and return home along side the only two remaining hearts? Or will they perish and join the rest forever?