The story begins in the Land of Departure, where a trio of friends and Keyblade wielders—Terra, Aqua, and Ventus (Ven for short)—train under the Keyblade Master Eraqus to become Keyblade Masters themselves. Terra and Aqua take an exam to achieve the Mark of Mastery, but while Aqua passes, Terra fails due to the abundant darkness in his heart. Shortly after the test, another Keyblade Master, Xehanort, disappears, and a hoard of mysterious creatures called the Unversed begin to appear throughout the worlds. Eraqus sends Terra to destroy the Unversed and find Xehanort. Ven is goaded into following Terra by Vanitas, Xehanort's enigmatic apprentice, against Eraqus's wishes. To watch over Terra and bring Ven back to the Land of Departure, Aqua is sent out to track her friends down.
Upon arriving in several worlds, Terra, Aqua, and Ven encounter various Disney characters, fighting off many Unversed along the way. Among the characters they meet is Mickey Mouse, who is also on a mission. Terra ends up attracting the attention of several Disney villains, who offer him the whereabouts of Xehanort in exchange for assistance in their own agendas. Terra reasons that 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 Xehanort out. 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 the heart of her royal blood, 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 will not fall into the villains.
Terra and Aqua eventually find themselves at Destiny Islands where they encounter 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 and asks Sora to protect Riku from Darkness. Ven, meanwhile, comes across Xehanort, who reveals his intentions to use Ven's heart to create a final Keyblade known as the Χ-Blade which he also says Master Eraqus had known all along. Ven returns to the Land of Departure to confront Eraqus, who tries to erase Ven to prevent Xehanort's plans from succeeding. However, Terra, who had been sent there by Xehanort, attacks Eraqus and teleports Ven to safety to protect him. Upon his defeat, Eraqus is vanquished by Xehanort, who tells Terra to go to the Keyblade Graveyard and leaves the Land of Departure to be consumed by darkness. At Destiny Islands, Ven once again encounters Vanitas and learns of his past: Ven was once Xehanort's apprentice, but his heart was deemed too weak for Xehanort to use him as he intended. As with Terra before him, Ven is instructed by Vanitas to go to the Keyblade Graveyard. Aqua, in the meantime, learns of Eraqus's defeat 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. 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 progress. In the following battle, the enraged Terra unleashes the darkness in his heart and is possessed by Xehanort, but his strong will remains within his discarded suit of armor, which comes to life as the Lingering Sentiment and defeats Xehanort, knocking him unconscious. Meanwhile, Vanitas fuses with Ven to create the Χ-Blade and battles Aqua alongside Mickey while Ven fights him from within himself. Aqua shatters the Χ-Blade as Ven destroys Vanitas, but his heart became 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.
Mickey saves Aqua and Ventus and brings them to Yen Sid's tower, where Aqua learns that Ven has lost his heart and will not be revived until his heart returns. Aqua brings Ven back to the ruins of the Land of Departure, where she unlocks the Chamber of Waking and creates Castle Oblivion from the ruins using Master Eraqus's Keyblade. After leaving Ven within the chamber, Aqua goes to Radiant Garden where she encounters and defeats Xehanort, who has lost his memories but still possesses Terra's body. Terra then attempts to stop Xehanort from within by sending himself into the Realm, but Aqua dives into it and saves Terra from disappearing with her armor and Keyblade. She ends up getting trapped in the Realm of and beginning a journey to return to her friends. Xehanort is discovered and taken in by Ansem the Wise, ruler of Radiant Garden, as his apprentice. Ven's heart of light then finds its way to Sora is able to merge with his heart.
After the events of Kingdom Hearts II, Aqua comes across Ansem in the Realm of Darkness with only a few of his memories remaining. He tells Aqua all about his encounters with Sora and that Sora is now the only one who can stop the worlds from being engulfed in darkness and save those connected to his heart, including Terra and Ven. Sora, having now read the letter from King Mickey, bids farewell to Kairi as he prepares to depart on another journey to save everyone who is connected to him.
Setting Further information: Universe of Kingdom HeartsBirth by Sleep is a prequel to the first Kingdom Hearts game, taking place ten years prior. Like the other games in the series, the player progresses through a collection of various worlds, based on various locales from the Disney animated features canon: Dwarf Woodlands from Snow White and the Seven Dwarfs; Enchanted Dominion from Sleeping Beauty; Castle of Dreams from Cinderella; Olympus Coliseum from Hercules; Deep Space from Lilo & Stitch; Neverland from Peter Pan (featuring for the first time most of the main island setting new to the series rather than Captain Hook's pirate ship and Big Ben from the first game and Kingdom Hearts 358/2 Days); and Mysterious Tower from Fantasia. The game also features various new worlds specially created for the series by Square Enix such as the Land of Departure and Radiant Garden. Disney Town, which is the town that surrounds the castle grounds of Disney Castle, serves as the game's minigame world. Worlds such as Destiny Islands and Castle Oblivion also appear in the form of cutscenes, while the 100 Acre Wood from the Winnie the Pooh franchise, which had appeared in most other games of the series, only appears as a Command Board in Birth by Sleep.
The three main characters are Terra, Aqua, and Ventus, a trio of close friends and apprentice Keyblade-wielders under Master Eraqus who aspire to become Masters themselves. Another Keyblade Master, Xehanort, and his mysterious apprentice, Vanitas, serve as the game's antagonists. Other characters from the previous games return, such as Sora, Riku, and Kairi, though they are younger than their original incarnations and given less prominent roles. A number of characters who later come to form Organization XIII also appear, particularly the first five apprentices of Ansem the Wise (Braig, Dilan, Even, Aeleus, and Ienzo) as well as Lea and Isa. As with the Heartless in Kingdom Hearts and the Nobodies in Kingdom Hearts II, Birth by Sleep introduces a new type of enemy: the Unversed, beings who oppose human life and feed off of negative emotions, created from those who are not well-versed in their own existences.
As with the other games in the series, Birth by Sleep features an extensive cast of Disney characters. The most prominent of these characters is King Mickey, who serves as a student under Yen Sid at this point in the series chronology. Donald Duck and Goofy, who served the main protagonist Sora in most other games, appear in smaller roles. Most other Disney characters remain in their respective worlds regardless of their overall importance to the story in previous installments, such as Maleficent and Pete, who served as major antagonists between Kingdom Hearts and Kingdom Hearts II, and a number of the Princesses of Heart. Some characters (such as Experiment 626, Prince Phillip, and even King Mickey) occasionally fight alongside the player characters in battles as partners similar to the other games, providing assistance and performing combos to help the player proceed. However, they do not follow the player around constantly like Donald and Goofy in the other games. Unlike the other major installments, which also featured casts of various characters throughout the Final Fantasy series, Birth by Sleep only features two of these characters: Zack Fair from Final Fantasy VII, and Moogles, from several of the Final Fantasy games. As Birth by Sleep is a prequel of the first Kingdom Hearts title, the staff decided to add Zack to the title as he is protagonist from the Final Fantasy VII prequel, Crisis Core: Final Fantasy VII.