This is the one game I haven’t played, so I’m just going to quote the in-game summary:
"Master Xehanort had decided to abandon his pupil Ventus on the Destiny Islands. The boy’s heart was fractured - useless. But his heart was saved when a 'new heart' found Ventus and helped him fill the void.
Several years passed, and Ventus forgot what had happened. He had found a new home in the Land of Departure, a sacred ground defended by a long line of Keyblade Masters. There, he met two other youths named Aqua and Terra, and together they trained with Master Eraqus to become Keyblade Masters.
Both Aqua and Terra should have been named Keyblade Masters, but Master Xehanort intervened, convincing Eraqus to test his pupils with a 'Mark of Mastery' examination. While Aqua passed the test, Terra did not, creating a rift between him and his friends.
Shortly after came tidings of 'Unversed' - strange creatures that thrived on negativity - and Master Xehanort had broken off all communication. Terra left to solve both mysteries; Ventus ran away to be with his friend. As for Aqua, Eraqus tasked her with keeping Terra under surveillance and bringing Ventus home as soon as possible.
Thus, their journeys began.
During Terra’s travels, Master Xehanort whittled away at the young man’s ethics, convincing him that darkness and light were equally important, and that a balance must be kept. Elsewhere, Ventus met Mickey, another trainee, and found himself crossing Keyblades with a mysterious masked boy named Vanitas.
The three friends were united in Radiant Garden, but too much had changed. Terra felt betrayed by Eraqus and Aqua, and retreated deeper in Xehanort’s clutches; Ventus sided with Terra and ran off again; and Aqua was left to deal with a mixture of guilt, concern, and love.
Guided by fate, perhaps, Terra arrived at the Destiny Islands and met Riku. He decided to bequeath the power of the Keyblade to this little boy, who reminded him so much of himself.
Some time after, Aqua visited these same islands and discovered what Terra had done. She, too, found a kindred spirit in Riku’s friend Sora, and told the boy to stick by his friend and guide him even through the darkest of times.
Ventus had made a discovery of his own. While rescuing Mickey, he was confronted by Master Xehanort, and learned of his connection to a weapon called the 'X-blade'.
Ventus returned to Master Eraqus in search of answers about his past. But the truth was a disaster waiting to happen, and Master Eraqus had no choice but to lock his pupil’s heart away to keep that disaster contained. Terra arrived just in the nick of time and battled his master to protect his friend.
But this was exactly what Master Xehanort had hoped for; Terra watched in horror as his new 'mentor' struck the weakened Eraqus down. He had been fooled.
Terra tried to move Ventus someplace safe - the Destiny Islands. But Vanitas, the boy with the mask, was already there. 'Join with me; become the X-blade,' he said. And then Ventus remembered the truth; Vanitas was a part of him - the darkness which had been extracted from his heart.
Led by destiny, Terra, Ventus, and Aqua assembled in the Keyblade Graveyard.
In the distance, Master Xehanort and Vanitas appeared. A violent clash ensued, and at the end of it, Terra’s body was possessed by Xehanort. Meanwhile, Ventus, having been forced into a clash with his counterpart Vanitas, chose the destruction of them both to prevent the X-blade from being forged.
In the aftermath, Mickey was able to rescue Aqua and Ventus, but Ventus would not wake, and Terra was nowhere to be found. Aqua’s journey was not over.
Aqua recalled her master’s teachings. After finding a safe place for Ventus to rest, she locked the Land of Departure - which would eventually come to be known as 'Castle Oblivion.'
Ventus’s heart had been nearly snuffed out, so he retreated to the safety of Sora’s heart - the same 'new heart' that had saved him once before.
Aqua found and confronted Xehanort, who had possessed Terra’s body. After a long fight, Aqua dove into the darkness and sacrificed herself to usher Terra back into the realm of light.
Aqua wandered the realm of darkness for a decade before meeting a stranger there. This stranger told her about a certain boy who had become the savior of all worlds. When Aqua heard the boy’s name, tears filled her eyes, and her face beamed with hope…"