Kingdom Hearts Unchaide X is situated before the Keyblade War, which is what established the organization of the Kingdom Hearts universe that we see in the original game. In this mobile game, we play as a Keyblade wielder and join one of the five factions led by Keyblade Masters and we fight for control of the limited light existing in the world.
So far, other than that, there isn’t much of a story. You follow a small story in each world that you visit, but there doesn’t seem to be a big main story like the other games. I’m currently at quest #133 and I’ve visited Daybreak Town, Dwarf Woodlands, Wonderland, Agrabah and Olympus Coliseum. They seem to keep adding quests so I’m not sure if there’s an end. I will keep playing and if I ever reach the end and it seems like there was a bigger story to this mobile game, I’ll update you. So far, it’s just the adventures of a Keyblade wielder fighting for control of the limited light left in the world.