Plot:
This story is independent of the Kingdom Hearts games.
The main characters, [insert here], [insert here], and [insert here] have been transported from their worlds to the world of Mesotamia. [insert here] becomes the wielder of the keyblade. The main characters eventually meet some people in Mesotamia, and the people inform them on what has happened the reason they are there. After being informed about the the Heartless, the keyblade, and the fact that there are other worlds, a wizard of Mesotamia named Wiwoka arrives. He provides the main characters with even more information, and tells them that they must travel with him to save the other worlds and defeat the Heartless.
If you need information about the keyblade, the heartless, or other specific details regarding Kingdom Hearts then look here: Kingdom Hearts Wiki