0:07 - 0:13 - The Forerunner Shield World
Many have speculated that this is Onyx or Reach. But it’s confirmed that the location has never been seen in Halo canon.
0:14 - 0:33 - The Chief’s arrival + Exploration of the Shield World
Not much is going on during this part.
0:34 - 0:38 - Reveals the Forerunner structures that dangle from the Shield World’s shell. In case you don’t know what a Shield World is, it’s an artificial planet the Forerunners created to shield themselves from the blast of the Halo rings. How so? What you see in the trailer is insidethe Shield World. Which explains the Portal that the Chief entered on its exterior.
0:39 - 0:44 - The possible exit of the portal. How so? There seems to be much activity going on in the center of what we see in this shot. You see two creatures(That look like Sentinels) fly toward this area, so something’s gotta be going on. Possibly the Chief’s arrival?
0:45 - 1:00 - Just Forerunner outposts and other structures being shown. Nothing major.
1:01 - 1:07 - The most important part of the trailer. What you’re probably looking at here are UNSC drop pods and two pelicans on the right of them as they enter the same portal MC entered. Chief most likely set off some kind of beacon in the interior of the planet that caught the UNSC’s attention. So they send teams via drop pods and pelicans into the world to aid the Chief.
1:08 - 1:12 - Obviously a huge battle between the unknown enemy and UNSC. But the thing about this scene is that there seems to be a Halo ring in the background. Looks like we’re getting multiple settings.
1:13 - 1:21 - The UNSC ship that we saw dropping the drop pods and pelicans into the Shield World is destroyed. This ship is possibly The UNSC Infinity.
1:30 and on - The enemy.
Just some of my thoughts 