The Draenei were one of two new races introduced in the game's first expansion, The Burning Crusade, and they were a fitting addition to the Alliance, since it was by fleeing the expansion's "Big Bad," Illidan and his demon forces that were invading their home planet, that they came to be in the game. They escaped their dying planet by spacecraft, but as tends to happen, it suffered too much damage and they crash-landed on Kalimdor, the western continent of the world.
Luckily there was an inanimate carbon rod on board
After helping the survivors find their way back to the wreck and do some recon work, you discover that some of your ship's debris has contaminated the nearby environment, creating some pretty vile monsters. Your leaders decide that you need to make things right, and they send you off to kill the monsters and stop the contamination.
As you progress, it becomes apparent that the local population isn't friendly. You start butting heads with some blood elves, (the other race added in the Burning Crusade), as they attempt to summon some demons to trash the place, and have to get involved with stopping them.
Before their diabolical boy band plan comes to fruition.