Kingdom hearts is the collection of all hearts and worlds: essentially existence itself. Trying to access it from the inside is a weird problem to face, but if you succeeded you would have control over reality. In order to get this power, a bunch of keyblade wielders (keyblades are weaponized hearts) went to war with each other over it, causing untold destruction and ripping the world into many worlds. Learning about this story, Xehanort wishes to form the xi-blade: the ultimate keyblade that can open Kingdom Hearts, but to do this he must have light mix with darkness in a great clash. He first tries splitting a young keyblade warrior named Ventus into light and dark to accomplish this, but he fails. Being old and running out of time, he takes over the body of a young keyblade warrior named Terra. Aqua is irrelevant. Xehanort continues his research and comes up with a very convoluted plan to create the Xi-Blade, he will split his own heart between 13 vessels and clash with either 7 hearts of pure light: the princesses, or with 7 Guardians of Light: keyblade warriors who have overcome their own darkness. To facilitate this he splits his own being into a Heartless: his heart consumed by darkness, and a Nobody: his husk that is devoid of heart, and each get to work preparing for the plan. Ansem, seeker of Darkness, Xehanort's heartless, goes back in time to explain the plan to his past self, then returns to the present. He wants to gather the 7 princesses of Heart, and to get the Heartless on his side, promises to give them access to the full darkness of Kingdom Hearts and attempts to create an artificial one using the hearts of worlds. But he is thwarted by Sora and is destroyed. Xemnas, Xehanort's Nobody, gathers 13 nobodies that can serve as vessels for Xehanort's heart. He uses the hearts of people to create an artificial Kingdom Hearts by baiting Sora into destroying Heartless, which he intends to use as the tool by which Xehanort will transfer his heart fragments. This is also foiled. With Ansem and Xemnas destroyed, his original self is reformed, and Xehanort gives on last go at orchestrating the clash of 13 darknesses with 7 lights, and this time he succeeds. Acquiring the Xi Blade, Xehanort unlocks Kingdom Hearts and begins to unleash its darkness to reset the world so that he may reform it in his image, but Sora is able to stop him. Realizing that his vision for the world is flawed, Xehanort finally gives up, surrendering the Xi Blade to Sora. Sora then tries to find Kairi in the past, who Xehanort had destroyed, to save her, but by doing this crossed a line he should not have and passed into the other side: The Waking World, revealing the world of Kingdom Hearts to be just a dream