The first big mystery to NBC's Day One series has been solved: what attacked the Earth and caused Doomsday? Well, some sort of big-branched alien thing. Check out the first extended trailer for Jesse Alexander's series.

So are those two oddly calm people aliens, who were once attacked by the giant under-the-table aliens before? I'm intrigued. We'll see what happens next on NBC. Here's the official synopsis, still keeping us pretty much in the dark:

Day One chronicles a global catastrophe from the perspective of the residents of a southern California apartment complex. In the wake of a mysterious event that has shattered lines of communication and (literally) changed the shape of the world, the survivors must work together to both rebuild and save the Earth from a menace that might have already consumed countless other worlds...

[Via Quiet Earth]