Two children, a boy and a girl, are lying in a clearing amongst blades of grass, staring up at the night sky.
Boy: You ever wonder what's up there?
Girl: Like what?
Boy: Maybe someone up there is wondering what it's like here.
Girl: I guess. Do you think we'll ever meet them?
Boy: I hope so. Don't you?
We see the Master Chief's helmet now in same clearing where the two children were a moment before. Screams, noises of combat and what sounds like incoming artillery fire can be heard.
In a split second, the clearing and blades of glass are replaced by a different scene: daytime, brown sand in the foreground, with scattered clumps of grass. The only element that remains from the previous shot is the Master Chief's helmet. Some kind of artillery shell explodes in the distance, throwing up a plume of sand.
The camera, seemingly looking through the Chief's eyes, scans upward as blurry craft in the distance fly overhead. Some look like Covenant Banshees. The Chief's gloved hand paws at the sky.
The girl's voice, saying "Do you think we'll ever meet them" is heard a second time. A male voice, seemingly from a combat radio, can also be heard but the words are difficult to make out.
The Chief turns to pick up his helmet at put it on. The visor appears to be cracked near the right side. The reflection of his gloved hand can be seen in the visor, but he turns it over before putting it on, so nothing else is clearly visible reflected on the visor.
UPDATE: After viewing an HD version of the ad, I can say that the visor is merely dirty on that side, and not actually cracked.
As the camera focuses in on the darkness inside the Chief's helmet, the screen goes black.
When the screen fades in from black, the camera is now above the Chief and rotating around him 180 degrees to show his surrounding.
The girl's voice, almost a whisper, says "Time to go."
Another female voice, possibly Cortana's, says "Chief, leave me." Or possibly "don't leave me". Or possibly just "leave". Or possibly "believe me" which puts a really and truly different spin on things.
The camera settles, eye level to the ground, in front of the Chief's assault rifle. In the background, behind the Chief and to his left, is an overturned and damaged Warthog with its left front wheel in the air, still spinning.
The male voices on the radio get louder and louder. Finally one is clearly audible, as the camera changes to a position above and behind the Chief, moving downards. He is slinging the assault rifle onto his back. What look like plasma fire from Wraith tanks can be seen rising into the air far in the distance.
Male voice 1: "Marines, move it, now!" (alternatively this may be "marines, fall back now".)
Male voice 2: "Any sign of the Chief?"
Male voice 3: "Negative, sir. I think we lost him."
The camera moves in for a closeup on the Chief from a low angle, looking up at him from below him and to his right.
Master Chief: "Not yet."
The Chief looks down, revealing a cylindrical object, larger than a frag or plasma grenade, in his right hand. He depresses a button on the top of the device with his thumb, revealing orange stripes along the side of the device. It makes a whirring sound, and a surface protrudes from the bottom of the device.
The Master Chief spikes the device into the sand in front of him, and it makes an electronic sound as he does so. At the same time, we are able to see that he is holding a pistol in his left hand. In the distance camera right, we see more incoming plasma streaks. In the near background camera right is another overturned human vehicle, possibly a Warthog or a Mongoose ATV.
A semispherical shield comprised of hexagonal cells forms around the Chief, just as one of the plasma bolts explodes near him. The explosion sends the wreckage of the overturned vehicle flying out of frame, and a dust cloud obscures the camera's view.
The shield dissipates, and when the dust clears, the camera is looking directly at the Master Chief as he begins to run directly towards the camera. The strains of the famous Halo theme begin to rise in the background.
The camera changes position to view the Chief in profile as he runs, removing the assault rifle that was on his back and holding it in his right hand. Another explosion nearby seems to throw him off balance but he continues running.
The camera is now behind the Chief at some distance, and we can see that the Chief is running directly towards a cliff edge. As the Chief nears the edge, we can see at least five Covenant Wraith tanks below. There are also fourteen figures in front of the Wraiths. They appear to be Brutes in some kind of heavy armor. Some dive for cover as the Chief jumps in amongst them.
The camera moves to a position on the ground, looking up at the Chief as he jumps down off the cliff. As his feet hit the ground, the screen blacks out and the "Halo 3" logo appears, followed by "Finish the Fight 2007", the Xbox 360 logo, and the halo3.com URL.