Long before Sora was chosen by his Keyblade, the safety of the worlds lay in the hands of the true Keyblade Masters. Three youths — Terra, Ventus, and Aqua — have been training long and hard under Master Eraqus to prove that they, too, exhibit the Mark of Mastery. However, they will soon find themselves in the middle of a crisis affecting worlds far beyond their own, just as another Keyblade Master, Xehanort, goes mysteriously missing.