Fire, Air, Water, Earth. These are the four elements that define the world you live in. One person in the entire world is a master of all the elements. That person is the Avatar. The current Avatar was announced recently, but not before she was able to make some enemies. Her name is Kyoshi.
Some time before the team met, a man in a bird mask met them and promised them power. The earliest was a year ago, while the latest was just a few weeks ago. All he said was "Meet in the Evergreen Forest in the Earth Kingdom." The date was given, the pieces were in place, the masters at their boards, and the game ready to be played.
