Story so far

Three Keyblade apprentices, Terra, Aqua and Ventus, train under Master Eraqus to one day become Keyblade Masters. However, when Aqua is named Keyblade Master after passing her Mark of Mastery, Eraqus's fellow Keyblade MasterXehanort mysteriously disappears as monsters called the Unversed begin spreading throughout the worlds. Terra is sent to defeat the Unversed and locate Master Xehanort. Ventus is goaded into following Terra by Xehanort's enigmatic apprentice, Vanitas, and Aqua is instructed by Eraqus to bring Ventus back home. Along the way, Terra slowly begins to lose control in the darkness inside his heart, but finds solace with Master Xehanort, who appears to understand him.

Terra crosses paths with a young boy named Riku and chooses him as his successor. Ventus befriends fellow Keyblade apprentice, Mickey Mouse, and crosses paths several times with Vanitas. He eventually learns that Vanitas is actually the embodiment of the darkness inside him, extracted by Master Xehanort as part of his plan to forge the legendary χ-blade, capable of unlocking Kingdom Hearts itself. Ventus confronts Master Eraqus about this, who attempts to destroy him to stop the χ-blade's creation, but falls at the hands of Terra and Master Xehanort, the latter whom reveals his true intentions afterward.

Terra, Aqua and Ventus eventually battle both Master Xehanort and Vanitas in the Keyblade Graveyard, where Ventus and Terra's fates are sealed: Terra converted into Xehanort's vessel while his mind took residence in his discarded suit of armor, Vanitas merging back into Ventus and taking brief control over his body in a fight with Aqua and Mickey physically before the χ-blade he used is imperfect as the fusion is revealed to be. In his mind, Ventus sacrifices himself to destroy Vantias. However, as Aqua placed Ventus comatose body to rest inside the Land of Departure, reformatted as Castle Oblivion, Ventus's heart makes contact with the heart of a young boy named Sora, entering the boy's body.

While looking for Terra, Aqua finds Xehanort in the Radiant Garden, where he was sent following the χ-blade's destruction, and battles him in an attempt to rescue Terra. As Xehanort begins to sink into the Realm of Darkness, Aqua follows and sacrifices her armor and Keyblade to save Terra's body while she remains in the darkness. Xehanort, now with amnesia, is found by the sage king of Radiant Garden, Ansem the Wise, and taken in as his apprentice. However, with the aid of his ally Braig, Xehanort unknowingly embarks on a back-up part he arranged if his plan had failed. By that time, Mickey passes his Mark of Mastery under Yen Sid and is awarded the title of Keyblade Master.