As we get closer to the release of Kingdom Hearts III we're taking a look at the rest of the series.
Technically, the first game in Kingdom Hearts chronology is Kingdom Hearts X, a mobile game that tells the story of the Keyblade War, an event that took place a long time before the events of the first Kingdom Hearts game. However, the important part of Kingdom Hearts chronologically starts with 2010's Kingdom Hearts: Birth By Sleep. 10 years before Kingdom Hearts we meet three Keyblade users, Terra, Aqua, and Ventis.
A new sort of evil has been seen throughout the worlds called the Unversed. Terra and Aqua are sent to search out the cause of the evil. Meanwhile, Ventis, being told by a random stranger that his friend Terra will never be the same again, runs off as well.
What makes Kingdom Hearts: Birth by Sleep unique, and a lot of fun, is that players then have the ability to play through the same story with all three characters. The three characters cross paths with each other at various points, but you only ever see the story from one character's perspective at a time.
This makes it not only necessary to play through the game as all three characters in order to get the whole story, but a lot of fun, as the first campaign will leave you with many questions about what the other characters did that you'll answer as you play through a second or third time. You'll be waiting with anticipation as you await an event for a character that you know is going to happen because you saw the results or heard about it from another story.
In addition to new playable characters, Birth by Sleep is full of new places to go exploring as well. You get to play through an entire collection of Disney worlds that we haven't seen before. Snow White and the Seven Dwarfs, Cinderella, and Sleeping Beauty all make appearances along with later Disney films like Lilo & Stitch. Each playable character arrives in each world at a different time and/or in a different place, so while all of the Disney Princess' worlds tell a story more or less identical to the classic Disney movie, you have to play all three characters to get the whole story.
Each character also plays differently. Terra is big and forceful, a Keyblade tank, if you will. Aqua's abilities are more geared toward magic than combat. While Ventis is a good balance of the two, the closest to Sora of the three.
And speaking of Sora, the events of Birth by Sleep are more than directly related to the main games. While most of the other spinoff games in Kingdom Hearts aren't strictly necessary to get the most important story beats, Birth by Sleep is absolutely vital to the larger story. You learn a great deal more about Xehanort, the ultimate big bad of the series, who is expertly voice acted by the great Leonard Nimoy (incidentally, the other Keyblade master of the story Eraqus, is voiced by Mark Hamill in a perfect, and likely intentional, bit of sci-fi franchise combat).
Xehanort is on a quest to reforge the X-blade (here, using the greek letter X, so it's pronounced key-blade) because he wants to restart the Keyblade War, believing that darkness and light should be in equal balance, rather than seeing darkness consumed by light. He very nearly succeeds, but the combined efforts of the three characters puts the weapon, the protector of Kingdom Hearts, out of his reach.
You also do see Sora, Riku and Kairi and you begin to learn how they became part of this story at all. The three characters here and the three characters of the main games are literally linked together in the story. Terra, like Riku, is constantly battling the darkness inside him.
One benefit of the fact that Birth by Sleep is a prequel to everything that came before it, the story is pretty self-explanatory and clear on its own. While there are references to the stories that will come, understanding them isn't necessary to enjoy the story of this game.
While Kingdom Hearts: Birth by Sleep was well reviewed and also a solid selling game in its day, I can't help but wonder how many people didn't get a chance to play it when it was new because of the platform where it could be found. While the PlayStation Portable wasn't a bomb of a handheld console by any stretch, it never did nearly as well as the Nintendo DS, which was also the console where more Kingdom Hearts games were released.
I certainly never touched this game before getting a chance to play it in the Kingdom Hearts 2.5 ReMix. I'm certainly glad that the game is available to play in full now. While all the spinoff games add something to the story, and many have tried to do something unique with gameplay, Birth by Sleep is the only one that to me really felt like another Kingdom Hearts game. Even though you play as other characters, it still fells like the real companion to the two console titles.
And from everything that we know going into Kingdom Hearts III, the events of this game will be vital to the end of the story. Both of the two games in the franchise that take place after the end of Kingdom Hearts II (we're getting there next) deal with the characters from Birth by Sleep in some way. The game also ends on something of a cliffhanger for the characters, so their story obviously isn't over yet.
Kingdom Hearts: Birth By Sleep 0.2 - A Fragmentary Passage, was released as part of Kingdom Hearts HD 2.8 Final Chapter Prologue and it acts as both a playable epilogue to Birth by Sleep and a prologue to Kingdom Hearts III. It's the final set of events that takes place chronologically before the events of the upcoming PS4 title. In that game you play as Aqua as she attempts to find a way out of the land of darkness she has found herself in.
So now, we have more or less the whole story between the beginning and the end of Kingdom Hearts II. There's just a couple of gaps to fill in and a bit more story to tell before we get to Kingdom Hearts III.