When it comes to the role-playing genre of video games, there are very few RPG series' and games as complex as Kingdom Hearts. What once started as a simple happy-go-lucky adventure of Sora, Donald, and Goofy traversing Disney worlds and uncovering the secrets of several mysteries has become a vastly complex overarching story that can only be best understood in its entirety.
The thing that makes it difficult to absorb everything Kingdom Hearts has to offer, however, is that it tends to span across almost as many consoles as there are titles. While collections such as Kingdom Hearts: The Story So Far and the HD Remix games do a good job of making this easier, how exactly the timeline works can be tricky to keep track of, especially with time travel added into the mix. Thankfully, the series can be condensed down to important moments, and it's there that the timeline is a bit more understood.
Before Sora: Kingdom Hearts X, Union Cross, Dark Road, and Birth by Sleep
The Kingdom Hearts series interestingly starts off with its mobile game, Kingdom Hearts X [Chi] Unchained, which puts players in the original Realm of Light as their own Keyblade wielder avatar. It's through the events of X Unchained that Kingdom Hearts X Back Cover unfolds, as the Union Leaders search for the traitor among them in order to stop the Keyblade War.
They, and a girl named Strelitzia who died trying to prevent said fate, failed. The Keyblade War leads the Realm of Light to split into the smaller Disney worlds. However, Union Leader Ava's chosen Keyblade Wielders save the player before they can perish as part of the war, starting the events of Kingdom Hearts Union X, the updated version of the mobile game.
In Union X, the player character has forgotten the Keyblade War has ever happened with a Data world, and works under Ava's newly chosen Union Leaders, one of which being a boy named Ventus. Eventually, the Data world becomes corrupted, and these new Union Leaders are tossed into different parts of the Kingdom Hearts timeline when escaping. Dark Road, Xehanort's story, plays out, and Ventus is eventually found and raised to be his apprentice before Kingdom Hearts: Birth by Sleep begins. Xehanort then tries to make the X-Blade, Ventus and his friends Aqua and Terra are separated, Ventus finding young Sora's heart and choosing to stay with him.
After Sora: Kingdom Hearts Through KH: Melody of Memories
Aqua falls into the Realm of Darkness and spends Kingdom Hearts 0.2: Birth by Sleep – A Fragmentary Passage suffering for almost the entirety of each Kingdom Hearts game after Birth by Sleep. Years later, after Ventus found his heart, the story of Kingdom Hearts begins proper with Sora gaining his Keyblade as Destiny Islands falls to the Heartless.
After Kingdom Hearts, the timeline grows to be a bit more complex; Chain of Memories was released first, but Kingdom Hearts: 358/2 Days begins before it, with both games happening almost at the same time. As Roxas and Xion battle with Sora's power and memories, things go awry, and Roxas ends up sealed in a Data world to have Sora regain his memories back at the start of Kingdom Hearts 2.
After besting Ansem and Xemnas in Kingdom Hearts and Kingdom Hearts 2, this reforms Xehanort, who works in secret to form the Real Organization 13. In Re:Coded, King Mickey learns that Ventus, Aqua, and Terra need to be saved for the upcoming battle between the seven lights and 13 darknesses. This requires the Power of Waking, which Sora and Riku embark on a quest through Sleeping Worlds to obtain in Kingdom Hearts 3D: Dream Drop Distance.
Sora's journey is meddled with by the Real Organization 13, as they aim to turn Sora into a vessel for Xehanort. It's thwarted by Riku and King Mickey, but as a result, Sora is without the Power of Waking and has lost some of his Keyblade abilities. Almost all of Kingdom Hearts 3 acts as Sora's "training session" to regain the Power of Waking, which helps free Ventus and Aqua. In the battle against Xehanort, Kairi goes missing, and Sora chooses to use the Power of Waking to find her at a price.
In the ending of Kingdom Hearts 3: ReMind, Kairi is saved, but Sora disappears. Looking for answers, Kairi and Riku eventually find hints pointing to Sora being lost to a "fictional world" in Kingdom Hearts: Melody of Memory. With the help of the Fairy Godmother from Cinderella, Riku and Kairi are put in contact with Strelitzia, who reveals that Sora has most likely gone to a new world called Quadratum. She proceeds to help Riku start his journey that fans will most likely see in the next Kingdom Hearts game.