Its just a prequel. The enemies are called the Unversed, they are like the pre-Heartless. You will instantly notice the facial similarities of the characters in the game to the KH2 players. AKA Ventus, Aqua, and how they look like Roxas and......No Heart X. The whole storie isnt exactly explained, but a large part of it is. Also, it sets up another port game in the near future reguarding the Organization.

Also look for the appearances of a few of everyones favorites, with their hearts still intact.