The post-credits scene from Kingdom Hearts II was exciting, but also kind of bewildering. Who were these armor-clad figures? What is the "Keyblade War?" We got some of these answers in Birth by Sleep, a prequel game that takes place ten years before the original Kingdom Hearts. This game is widely considered to be a high point of the series, as it not only showcases a new side of the Kingdom Hearts universe, but also introduces some fan-favorite characters and sets up the eventual conflict that would take place in Kingdom Hearts III.

Birth by Sleep is focused on the trials and tribulations of a new set of three heroes: Terra, Ventus, and Aqua. The trio are apprentice Keyblade wielders training under Master Eraqus (whose name is an anagram of Square, and whose appearance is based on Hironobu Sakaguchi, so... well played, Square Enix) at the Land of Departure in an effort to become full-fledged Keyblade Masters. However, on the day of Terra and Aqua's Mark of Mastery test, they are paid a visit by one Master Xehanort, who manipulates the darkness in Terra's heart and causes him to fail his test before departing. Before long, the group receives word that the worlds are under attack from a malevolent force known as the Unversed (basically proto-Heartless, and this game's new enemy type). At Master Eraqus' behest, Terra sets forth on a mission to investigate the Unversed invasion and to track down the now-missing Xehanort, with the hope that should he prove his worth, he will be reconsidered as a Keyblade Master. Ventus, meanwhile, encounters a mysterious masked figure known as Vanitas, who tells him that the Terra he knows will soon be gone forever. Spurred by this, Ventus runs away from home in pursuit of Terra. Because of this, Aqua is sent in pursuit, both to bring Ventus home and to keep an eye on Terra should he succumb to the darkness.

Birth by Sleep is unique in that is has three separate campaigns, one for each protagonist. The player can choose to play through Terra's, Ventus', or Aqua's story campaigns in any order, and the trio's tales will occasionally intersect as they visit the same locales and uncover the sinister truths behind Master Xehanort and Vanitas. The adventure culminates at the Keyblade Graveyard, a wasteland filled with discarded Keyblades that once served as the site of a battle for the light within Kingdom Hearts: this became known as the Keyblade War, and Xehanort is hoping to recreate the legendary X-Blade (that's X as in the Greek numeral, pronounced "key") in order to uncover Kingdom Hearts for himself. In order to accomplish this, he needs to unite the pure heart of Ventus with the dark heart of Vanitas, who is made up of the darkness that was excised from Ventus long ago. Unfortunately, Xehanort has grown old, and wishes to extend his life by finding a younger, more suitable host body to inhabit. As such, he has been grooming the darkness inside Terra, with the intent of having him lose control so he may inhabit the boy. Think of the relationship between Xehanort and Terra as somewhat akin to that between Palpatine and Anakin Skywalker: Xehanort masquerades as a mentor figure in order to turn Terra to the dark side.

Without getting into any more specifics... yeah, this doesn't end well for any of the heroes, and they all meet with tragic fates. Birth by Sleep probably has one of the best stories in the entire series: not only is it an interesting standalone adventure and a well-developed exercise in worldbuilding, but it also neatly sets up the events of Kingdom Hearts and even sows the seeds for what will (eventually) come to pass in Kingdom Hearts III. The core cast is clearly made up of foils for the main Kingdom Hearts trio, with Terra standing in for Riku, Ventus for Sora (and Ventus even looks exactly like Roxas for... reasons), and Aqua for Kairi, but they're all likable characters with unique personalities and struggles of their own. Terra's battle with the darkness inside him is particularly compelling, as he is led along by Master Xehanort and realizes his mistake far too late. Speaking of Xehanort, he's voiced by the late Leonard Nimoy, and it's a phenomenal performance, absolutely selling the character and his conniving nature. The voice cast in general does a really good job, and Nimoy is joined by Mark Hamill as the "good" Keyblade Master Eraqus. All in all, Birth by Sleep is an absolute must-play: while Kingdom Hearts II might be my personal favorite in the series, Birth by Sleep is at the top of the pile for sure.

Birth by Sleep also brings a number of innovations to the series' combat system. Players still use a command menu to select actions in real-time combat, but now, the menu is known as the "Command Deck." Pressing X will cause Terra, Ven, or Aqua to perform a basic attack, while the D-Pad and Triangle button allows players to cycle through their available commands. Players can equip different commands and arrange them in any order on their Command Deck: these commands range from elemental and healing spells, to consumable items, to special attacks such as a Keyblade throw or a dash attack. Players can even make more powerful abilities by fusing commands together with certain materials, adding a Shin Megami Tensei-esque fusion flowchart to the proceedings. It's a great system that allows for a lot of player customization. While level-ups and item drops often lead to new abilities, players can also play a Mario Party-styled board game from the main menu to nab some extra goodies. It's not essential to play this mode, but it's a fun distraction.

There are a couple of other wrinkles to Birth by Sleep's combat system: Shotlocks, Dimension Links, and Command Styles. When the player lands enough hits to fill a meter, they'll enter a different Command Style depending on what they have equipped. These Command Styles are powered-up forms that last until the meter is depleted, and end with a powerful finishing move. Since Birth by Sleep is largely a solo outing with whichever character you've selected, there are no party members: instead, players can use a Dimension Link to call an ally in battle, kind of like the Summons in earlier games. Finally, Shotlocks allow the player to lock on to an enemy for a multi-hit projectile attack, dealing a large amount of damage in the process. With all of these elements on hand, the combat in Birth by Sleep is fairly robust. There are some really challenging endgame bosses that require players to utilize their full arsenal effectively, and even more endgame content was added in the Japan-exclusive Final Mix edition of the game (which, fortunately, is included with the PS3/PS4 remasters).

There are no Gummi Ships in Birth by Sleep: players simply select the world they want Terra, Ventus, or Aqua to visit. Traversal in Birth by Sleep is a little more limited than past games, perhaps owing to the game's handheld roots: zones are usually little more than wide rooms or hallways with the occasional out-of-the-way treasure chest or collectible, which works well for short play sessions but isn't quite as well-developed as, say, the Pirates world from Kingdom Hearts II (which honestly might be the series' high point for level design). The heroes do unlock a few mobility-enhancing moves over the course of their journey, such as a high jump, glide, and air dash that makes traversing these zones a little more engaging, but other than finding the aforementioned collectibles (which are necessary for unlocking the epilogue chapter and true ending, by the by), there's not much use for them. Still, I can hardly hold that against an action RPG for the PSP, and the game looks and sounds really sharp despite its origins: Yoko Shimomura's score is as top-notch as ever, with tracks like Aqua's theme music and Vanitas' battle theme "Enter the Darkness" being particular standouts.

Should the players clear all three campaigns for Birth by Sleep and unlock the secret ending, they'll be treated to a special scene with Sora, Riku, and Kairi post-Kingdom Hearts II, as they prepare to set out on a new adventure: a clear tease for Kingdom Hearts III. But there are still a few more games to cover before we get there. We've got a digital world to visit, for one thing.