So you've decided to play Kingdom Hearts III. While you may have purchased this game to team up with Wreck-It Ralph and Buzz Lightyear to fight Sephiroth, your quest is frequently interrupted by people with anime hair yelling about darkness. If this is your first Kingdom Hearts game, or if you've ironically only played Kingdom Hearts I and II, you're gonna have a difficult time trying to make sense of it all.
Fortunately, there are options: You could buy Kingdom Hearts: The Story So Far, a $90 anthology, or watch hours of Kingdom Hearts YouTube videos. But there's also the path of least resistance, which is to simply read on, as we're not only going to provide a simplified explanation of Kingdom Hearts, but also answer the question: Does the plot of Kingdom Hearts even matter?
Mind you, Kingdom Hearts is deliberately convoluted. Series creator Tetsuya Nomura intentionally left vague some events and connections between characters, allowing players to use their imaginations to connect the dots, with canonical, and contradictory, explanations only appearing years later. Given the nature of the series, we're only going to be covering the touchstones. In fact, most of the Disney stuff – and literally all of the Final Fantasy content – really has no impact on the overall plot.
Let's establish the rules of the world: Kingdom Hearts hinges on hearts, which aren't literal organs, but basically emotional souls. Every heart contains both light and dark, reflecting one's morality.
Removing an individual's heart creates a Heartless, a being comprised of darkness. Conversely, the body that once housed the heart turns into a Nobody, which is basically a replicant. Destroying one's Heartless and Nobody restores the individual that created those two beings.
Additionally, memories in Kingdom Hearts can be implanted, fabricated, erased and even physically manifested. For instance, in Kingdom Hearts: Chain of Memories, Sora spends the entire game fighting through Castle Oblivion to retrieve his memories. At the end, however, Sora discovers that retrieving his memories means losing his recollection of Castle Oblivion. Therefore, everything you've done in Chain of Memories is basically for nothing. Fortunately, non-amnesiac characters are just as confused as the player throughout the series, essentially telling you not to worry about it.
The first Kingdom Hearts game focuses on three kids: protagonist Sora; Riku, the proverbial Sasuke to Sora's Naruto; and Kairi. Their home of Destiny Island is shattered when the Heartless suddenly invade through a tear in reality. Sora finds himself wielding a Keyblade, a key-shaped sword that can seal the tears and release hearts.
Destiny Island explodes, separating the kids. Sora is sent to Traverse Town, where he allies himself with King Mickey Mouse's highest mage and knight, Donald Duck and Goofy, to vanquish the Heartless across various Disney intellectual properties. Meanwhile, Riku teams up with Maleficent, who wants to control the Heartless. Kairi disappears, inadvertently housing her heart in Sora. To restore Kairi's heart to her body, Sora performs seppuku with a Keyblade, freeing his heart and Kairi's heart. Kairi, who is a DL Disney Princess, hugs the Heartless that Sora has become, restoring him to his original form.
The titular Kingdom Hearts is a giant, heart-shaped moon that houses the hearts of all worlds. To protect Kingdom Hearts, there is the χ-Blade, pronounced "Chi-blade," from which the Keyblades are derived. The χ-Blade allows one to unlock Kingdom Hearts; however, the blade was broken into seven pieces of light and 13 pieces of dark. Each game, thus, typically revolves around someone gathering the shards to rebuild the χ-Blade that unlocks Kingdom Hearts.
Kingdom Hearts is, essentially, like the Ark of the Covenant, with various antagonists having their own beliefs about what unlocking it actually does. Take Ansem, the true villain from the first game, who believes Kingdom Hearts is the source of all darkness, only to be immolated by light when unlocking its door.
Ansem's not totally wrong, however, as Kingdom Hearts is contained in the Realm of Darkness, a hellish void where time doesn't run correctly. Ten years before the events of the first game, in Kingdom Hearts: Birth By Sleep, the Keyblade Master Aqua sacrifices herself to the Realm of Darkness to ensure that her homie Terra escapes. Terra was possessed by series antagonist Xehanort at the time, so Aqua technically could've solved everything by doing nothing.