Master Cheif returns to a Covenant Dominated Earth on a mission to kill the final alien leader. Meanwhile, the Arbiter, Johnson, and Keyes form a loose alliance and escape from Delta Halo. The Covenant is ripped in civil war, and the Elites along with a handful of other alien races become sympathetic to the human cause. Delta Halo's impromptu dis-activation has brought all of the Halos to a "remote activation phase". They can be activated from a facility called the ark, which happens to be on Earth.

As the trailer begins, you can see a "grunt crossing" sign blowing in the wind.

Cortana:
I have defied gods and demons. I am your shield; I am your sword. I know you; your past, your future. This is the way the world ends.

Factual errors: Grunts and occasionally Brutes are heard referring to Master Chief and the Marines as "heretics", however as humanity was never a member of the Covenant, they couldn't be heretics. It is entire possible though, that the Covenant call anybody trying to halt their "Great Journey" heretics.