Kingdom Hearts is a crossover of various Disney properties based in a fictional universe. The series centers on the main character, Sora, and his journey and experiences with various Disney, Final Fantasy, The World Ends with You and Pixar characters. The heroes of the series clash against the various incarnations of the primary antagonist, Xehanort, throughout the series. Almost all characters and worlds of the Kingdom Hearts franchise are owned by The Walt Disney Company.
The series starts with Kingdom Hearts, where a 14-year-old boy named Sora is separated from his friends Riku and Kairi when their world, Destiny Islands, is invaded by creatures known as the Heartless. During the invasion, Sora obtains a weapon called the Keyblade that allows him to fight the Heartless. He soon arrives in another world, Traverse Town, where he meets Donald Duck and Goofy, two emissaries from Disney Castle sent to find the Keyblade wielder under orders from their missing king, Mickey Mouse. The three band together and travel to different Disney-themed worlds, sealing the hearts of the worlds to prevent them from being devoured by the Heartless as they search for their companions. Along the way, they encounter a group of Disney villains led by Maleficent, who are controlling the Heartless to capture seven maidens called the Princesses of Heart, who possess the power to open the door to Kingdom Hearts, the heart of all worlds. Though they eventually defeat Maleficent, the three discover a sentient Heartless named Ansem to be the true orchestrator of the worlds’ destruction. The three defeat Ansem and seal the door to Kingdom Hearts, but remain separated from Riku and King Mickey while Kairi remains home to await her friends’ return.
In Kingdom Hearts: Chain of Memories, Sora, Donald and Goofy’s search for Riku and Mickey leads them to Castle Oblivion,a fortress controlled by a mysterious group of non-existent “Nobodies” called Organization XIII, who use the power of a girl named Naminé to manipulate the trio’s memories. After defeating the Organization, the three are put to sleep for a year by Naminé to allow them to regain their memories, though they must lose the memories of their experience in Castle Oblivion.Meanwhile, Riku arrives in the basement of the castle and ascends to the surface with the aid of Mickey. Discovering Sora’s sleeping state, Riku helps Naminé keep him safe until he is awoken. In Kingdom Hearts 358/2 Days, a snag in the plan over the year forces Riku to capture Roxas Sora’s Nobody, who is born when Sora briefly becomes a Heartless in the first game—in order to enable Sora’s reawakening.
In Kingdom Hearts II, Sora and friends awaken from their sleep and resume their search for Riku and King Mickey, learning of the Nobodies and becoming re-familiarized with Organization XIII. Sora once again travels to many Disney-themed worlds and resolves issues caused by the Heartless and Nobodies, and Maleficent’s servant Pete. They reunite with King Mickey and encounter Xemnas, the leader of Organization XIII and Nobody of Xehanort, revealed to be the human form of the Heartless Ansem. The Organization’s plan is also revealed: they seek to regain their lost hearts by creating their own version of Kingdom Hearts from the sum of all the hearts released from the Heartless slain by Sora’s Keyblade. The three arrive at the headquarters of Organization XIII and team up with their friends. Ansem the Wise uses a device that dissipates some of Kingdom Hearts’ power, but it self-destructs, engulfing Ansem. At the top of the Castle that Never Was, Sora and his friends battle Xemnas. After Sora and Riku defeat Xemnas, they get trapped in the realm of darkness, but a letter from Kairi summons a gateway for them, and the two are reunited with their friends at their home.
Sometime later, Sora, Riku, and Kairi receive a letter from King Mickey. The letter, written by Mickey during the events of Kingdom Hearts Coded, describes the parts of their past that Naminé learned while restoring Sora’s memory. As they read the letter, they learn about the fates of three Keyblade wielders at the hands of Xehanort during the events of Kingdom Hearts Birth by Sleep: Terra, who is possessed by Xehanort; Ventus, who sacrifices his heart to prevent Xehanort from creating the legendary χ-blade, hiding it within a four-year-old Sora’s heart; and new Keyblade Master Aqua, who becomes trapped in the realm of darkness after sacrificing herself to save Terra. King Mickey also discovers that the destruction of “Ansem” and Xemnas has led to the reconstruction of Xehanort. To combat the new threat Xehanort poses, Sora and Riku take an exam to attain the Mark of Mastery that will allow them to become Keyblade Masters themselves.
The initial idea for Kingdom Hearts began with a discussion between Shinji Hashimoto and Hironobu Sakaguchi about Super Mario 64.