Kingdom Hearts Union Cross is a remake of the role-playing browser game Kingdom Hearts X[chi]. It's a prequel to Kingdom Hearts Birth By Sleep and Kingdom Hearts 1, taking place long before either game's time periods. KHX originally wasn't supposed to be canon, and only existed in Japan as a result. However, sometime during the game's lifespan, the director of the Kingdom Hearts series Tetsuya Nomura changed his mind and decided to make X's events important to the main story. This resulted in the creation of the mobile game Union Cross (originally called Unchained X). As Unchained X, the game recaps the events of X for international audiences. Once the game becomes Union Cross, it continues KHX's story past its original end.
Kingdom Hearts Union Cross has been going on for five years now, but this sixth year will be the game's last. A Kingdom Hearts 2020 update announced that Union Cross' story would be entering its final chapter soon. What happens in this chapter will officially tie the events of Union Cross into the main Kingdom Hearts timeline. Spoilers ahead.
Quest 1-3: Prologue
The game opens with a prologue of Kairi's grandmother retelling the story of the Keyblade War to Kairi as a child: Once upon a time, people lived peacefully in a world full of light, but they ended up fighting over it and bringing about a terrible darkness that destroyed everything. The only light to survive did so in the hearts of children, who rebuilt the KH universe as it is in Sora's timeline. Union Cross follows the journey of a Keyblade Wielder that lived before and after the Keyblade War. The protagonist is a customizable avatar of the player, so they don't have an exact identity. As such, they don't speak and instead mime their intentions through body language.
Once Kairi's grandmother finishes her story, Union Cross officially begins with the player character experiencing a unique Dive to Heart, which is a lucid dream where the dreamer can physically interact with the inner world of their heart or the hearts of others. This inner world is typically completely black, save for something called a Station, which is a giant pillar with a stained-glass top. These glass tops display images of the dreamer and what's most important to them.
In the case of the player character, however, they find themselves on a Station with glass displaying the symbols of the five Union Leaders — Keyblade masters that taught wielders during this era. The names of the leaders are Ira, Aced, Gula, Invi, and Ava, and they rule over the Unions of Unicornis, Ursus, Leopardos, Anguis, and Vulpes, respectively. The player chooses which Union they want to join, but the game heavily hints that the canon option is Ava, so that's who this recap will refer to. Upon selecting a Union, the player character unlocks the ability to wield a Keyblade and wakes up to a creature called Chirithy in Daybreak Town.
Chirithy explains to the player that the Keyblade is a weapon of light capable of destroying the physical manifestations of darkness called Heartless. Those who wield it have a responsibility to destroy Heartless and collect light or "Lux" for their respective Unions, which the Union Leaders will use to protect all of creation. During this era, new wielders empower themselves and their Keyblades with fragments of light called "Medals," which are trinkets that allow the player to tap into the powers and abilities of people from the future (Sora's timeline).
Chirithy then officially introduces itself and reveals that it was tasked by Master Ava to act as the player's guide throughout their journey. However, before they depart to other worlds, Chirithy has the player practice fighting Heartless in Daybreak Town. When the player finishes, Ava appears before them and tasks them with one more mission, which foreshadows a struggle the Union Leaders themselves will go through. Ava wants the player to weed out the darkness-users that hide amongst their ranks and collect Lux for selfish purposes. The player accepts, and then they and their Chirithy step through a portal to a new world, starting their adventure.
Quest 4-34: Dwarf Woodlands, Wonderland, Agrabah — First Visits
Arriving in Dwarf Woodlands, the player meets Doc and learns he got separated from his fellow Dwarfs, so they escort him through the Heartless-infested Dark Forest and reunite him with Happy, who's lazing in the Dwarf's Mines. Happy and Doc thank the player and claim they can find the rest of their gang on their own, so the player heads to Wonderland. Here they meet Alice and protect her from Heartless while she looks for the White Rabbit unsuccessfully. After receiving her gratitude, the player sets off for Agrabah and finds it under attack by Heartless.
After clearing out Agrabah with its citizens' help, the player helps Aladdin investigate rumors of a Wily Bandit Heartless sighting in the desert. This particular Heartless unintentionally kidnapped Aladdin's friend Abu, who tried to attack it when it was teleporting away. The duo investigates the sightings, but they turn out to be for a different Heartless. Not wanting to put the player in any more danger, Aladdin sends them on their way, but promises he'll ask for help in the future if he really needs it.
Quest 34 Epilogue: Nightmare Chirithy's First Appearance
The player returns to Daybreak Town to rest, and find themselves in the town's Fountain Square when they're approached by a Chirithy they believe to be their usual guide. This Chirithy congratulates them on their progress and gifts them with an item called the Power Bangle, which it explains will allow the player to increase their Medals' power more than ever before. The player's real Chirithy arrives when this mysterious one departs, and it's confused as to what's taken place. However, it opts not to alert the player to the fact that the Chirithy they spoke to was an impostor, which is a mistake that it ends up regretting later down the line.
Kingdom Hearts Union Cross is on Android and IOS.