Microsoft Halo Marketing is the best in gaming, period. I love that it's not just trailers but how they're always doing ARGs and extended media campaigns that involve the community. Those guys really know how to build real hype.
So my interpretation of the story based on the trailer:
Master chief has gone Rogue trying to hunt down and destroy the forerunners that are cropping up in various locations, recruiting his old team of Spartans for such a large task. ONI is blaming the MC for all these invasions (maybe because they have something to do with it). Locke is hunting down the MC to figure out what is going.