Difference in the series
- Some quotes in the series will be different from the game.
- More characters that are in the disney films that are incorperated in the Kingdom Hearts Birth by Sleep series are appearing in their particular world such as the other mice besides Jaq and Gus would be appearing in Castle of Dreams.
- There will not be any speech bubbles in the series, seeing that it is not the game series.
- There will be people dancing in the ballroom instead of just Cinderella and Prince Charming.
- There will be people walking around the kingdom in the Enchanted Dominion world, but once Ventus arrives in the world everyone will be sleeping.
- There will be people running around in the city of the Olympus Coliseum once the Unversed appear and attacks Hercules.
- There will be people walking around the city in the Olympus Coliseum world once Aqua appears in the world.
- In Olympus Coliseum people will be running around and scatter once the Unversed appear when Ventus visits the world.
- In the Olympus Coliseum games there will also be a crowd cheering for the participants.
- There will be aliens in the ship once Experiment 626 escapes in his ship with Ventus.
- There will be aliens trying to arrest Experiment 626 with Gantu.
- There will be more Lost Boys in Neverland instead of just Cubby and Slightly.
- Pirates will also appear on the ship trying to get Peter Pan.
- In Disney Town there will be more Disney folk wandering around playing games and all sorts of stuff at the Disney Town festival.
- There will be no split stories in this series, but it will show parts of their journey of what Terra, Ventus and Aqua are doing.
Kingdom Hearts Birth by Sleep begins in the Land of Departure, where three Keyblade wielders and close friends—Terra, Aqua, and Ventus (more commonly known as Ven)—train to become Keyblade Masters under Master Eraqus. To achieve this rank, Keyblade Apprentices must take the Mark of Mastery exam once they come of age. Terra and Aqua take this test together, but while Aqua passes the test, Terra is deemed unfit to become a Keyblade Master due to the abundant darkness in his heart. Afterward, a group of mysterious creatures called the Unversed begin to surface in other worlds, and another Keyblade Master, Xehanort, disappears without a trace. Terra is sent to destroy the Unversed and find Master Xehanort, who had earlier encouraged him to embrace the darkness in his heart if he desires the strength to become a Keyblade master. Ven, meanwhile, is goaded into following Terra by Xehanort's mysterious apprentice Vanitas, against Eraqus's wishes. Aqua is dispatched by Eraqus to survey Terra and keep him from falling into darkness, as well as to bring Ventus back to the Land of Departure.
Upon arriving in other worlds, Terra, Aqua, and Ventus encounter various Disney characters, new and old, fighting off hordes of Unversed along the way. Among the characters they meet is Mickey Mouse, apprentice of the sorcerer Yen Sid, who is also on a mission to destroy the Unversed. Terra ends up attracting the attention of several Disney villains, who offer him the whereabouts of Master Xehanort in exchange of assistance for their own dark agendas. Terra reasons that Master Xehanort is after the seven Princesses of Heart—maidens who lack darkness in their hearts—and encourages Aqua to help them strengthen the light in their hearts, hoping it will draw Master Xehanort out. However, Aqua and Ven grow increasingly concerned for Terra when they hear rumors of him committing sinister deeds throughout other worlds. The three briefly reunite at Radiant Garden, where Aqua meets Kairi, a young girl containing a strong and pure light in her heart, and helps strengthen her light. Aqua and Ven also confront Terra, who distances himself from them to keep them out of the way of his increasingly dangerous dark powers. Terra and Aqua try to convince Ven to return to the Land of Departure, but he refuses until he is certain Terra won't fall into darkness.
Terra and Aqua eventually find themselves at Destiny Islands, where they encounter two young boys, Sora and Riku. Terra sees potential within Riku and deems him worthy of one day wielding the Keyblade, while Aqua senses strong ties with Sora and Riku to Ven and Terra, respectively. Ven, meanwhile, comes across Master Xehanort, who reveals his intentions to use Ven's heart to create a final Keyblade known as the χ-blade, something Master Xehanort says Master Eraqus had known all along. Ven returns to the Land of Departure to confront Eraqus, who tries to erase Ven to prevent Master Xehanort's plans from succeeding. However, Terra, who had been sent there by Xehanort, misinterprets the situation and attacks Eraqus, teleporting Ven to safety. As soon as Terra realizes his error, Master Xehanort arrives and cruelly kills Eraqus and tells Terra to go to the Keyblade Graveyard, leaving the Land of Departure to be consumed by darkness. At Destiny Islands, Ven once again encounters Vanitas and learns of his hazy past: Ventus was once Master Xehanort's apprentice, but his heart was deemed too frail for Master Xehanort to use him as he intended, so he extracted the darkness in Ven's heart and used it to create Vanitas. As with Terra before him, Ven is instructed by Vanitas to go to the Keyblade Graveyard. Aqua, in the meantime, learns of Eraqus's death from Yen Sid, who tells her to go to the Keyblade Graveyard as well.
Terra, Aqua, and Ventus gather at the Keyblade Graveyard, where the remains of the mysterious Keyblade War are found. Master Xehanort appears with Vanitas and reveals his plan to use the χ-blade to unlock Kingdom Hearts and unleash the forbidden secrets of the Keyblade War onto the various worlds, as well as to use Terra's body as a new vessel for his own heart so that he may survive long enough to see his plans come to fruition. An epic battle ensues, with Xehanort freezing Ven, and Vanitas knocking Aqua unconscious. In his rage, Terra unleashes the darkness in his heart, inadvertently allowing Xehanort to unlock his own heart and successfully transfer it into Terra's body, which results in the birth of a new, younger Xehanort. However, Terra's mind continues to linger in his discarded suit of Armor, which comes to life and defeats Xehanort. Ven, meanwhile, manages to thaw himself before Vanitas can kill Aqua, and discovers his dark counterpart to be the source of all Unversed. Vanitas merges with Ven, possessing him and creating the χ-blade. Aqua awakens under Mickey's care, and the two fight the possessed Ventus while he battles Vanitas from within. Aqua shatters the χ-blade as Ven destroys Vanitas, bringing an end to the Unversed, though Ven's heart is lost in the process. The destroyed χ-blade unleashes a burst of light, which consumes everyone present and scatters them to the corridors of space, save for Terra's Lingering Will, which remains in the Keyblade Graveyard.
Mickey saves Aqua and Ventus and brings them to the Mysterious Tower, where Aqua learns that Ven has fallen into a deep sleep and will not awaken until his heart returns. Aqua brings Ven back to the ruins of the Land of Departure, where she discovers Master Eraqus's Keyblade, and uses it to unlock the Chamber of Waking and recreates the world as Castle Oblivion from the ruins. After leaving Ven within the chamber, Aqua goes to Radiant Garden where she encounters Master Xehanort still in Terra's body, but having lost some of his memories. Aqua then battles Xehanort who after the fight attempts to unlock his heart, losing his memories completely in the process. Soon after, Terra's body starts sinking into darkness, but Aqua dives into the darkness and saves Terra from disappearing. Realizing her glider won't be able to make it back through the portal in time, Aqua sacrifices herself by discarding her armor and using it to guide her glider, Terra-Xehanort in tow, back through the portal leaving herself behind and lost in the Realm of Darkness. Xehanort is discovered by Ansem the Wise, ruler of Radiant Garden, and goes on to become his apprentice. Ventus's wandering heart finds its way to young Sora, merging with the young boy's heart.