The plot? Well, how about Heroes vs. Aliens. In this case, the aliens are a species know as the Dominators. This event is based on the DC comics Invasion storyline, where the Dominators invade Earth over concern for the metagene influence in humanity. It turns out they were also trying to figure out how to gain the metagene capability for themselves and use it to attack other races.
Here is a full trailer for the event. It is great kickoff to the Holiday season!