They're Random, Baby!

Voiced-over backstory from Joseph Staten at Gamestock 2001

Hundreds of years from now, mankind is locked in a life-or-death
struggle with a group of aliens known as the Covenant. The Covenant
have destroyed our colony worlds... their forces... they're
unstoppable. To lead the Covenant away from Earth, a ship of human
soldiers makes a blind jump to a forgotten corner of the galaxy,
where they crash on Halo, an ancient ring-artifact, 10,000
kilometers in diameters, locked in orbit between an obscure planet
and its moon. And this... this is Halo [pans to show the ring]. And
THIS [pans back to the cyborg] - this is you. An elite military
unit, you are the last of your kind. Trained to fight with any
weapon, attack in any vehicle, you are Mankind's most sophisticated
piece of military hardware. Linking together with other survivors
from the crash, you begin a guerilla war against the Covenant army
that followed you to Halo. Whichever side, human or alien, which
first uncovers the secrets of Halo, will find something that will
alter the course of the war. Humanity's fate is in your big, green,
armored hands.