It is the year 2552. Humanity has long been at war with the terrifying alien civilization that collectively calls itself the Covenant. This monstrous conglomerate of warlike species sees Humanity as a form of heresy against their religion -- a religion based in the single -- minded belief that a Great Journey awaits its faithful and that this Journey can be embarked upon by firing an array of vast ringlike weapons scattered throughout the galaxy.
These immense terraformed rings, known as Halos, have been abandoned by their mysterious and long-vanished creators, the Forerunners.
This conflict, terrible and destructive though it is, has recently been complicated by a series of cascading events. One of these artificial worlds, these Halos, was discovered and subsequently destroyed by human forces, led by Spartan-117-- the Master Cheif-- the last fighting member of the legendary SPARTAN II military program and Humanity's last, best hope.
Conflict at a second Halo installation then ignited a galaxy-spanning conflagration as civil war erupted within the Covenant, driven by revelations that the Covenant leadership, the Prophets, had been deliberately deceiving its populace.
In truth, these Halos would not take the Covenant on a Great Journey but rather eradicate all sentient life in the galaxy--a last-- resort defense against the most terrifying menace the galaxy has ever known--and a menace the deluded Covenant has reawakened for the second time in a hundred thousand years.
That horror is the Flood--a sentient, all-consuming parasite that makes monstrous puppets of those it kills and will not stop until every last thinking creature is absorbed into its ferociously intelligent plan.
The second Halo has been activated, putting all the remaining installations in a dangerous state of emergency standby--they are now primed to fire. Worse still, the Covenant has discovered the long-hidden location of Earth and crushed almost all Human resistance as it seeks vital clues to the nature of the Halo array, some of which may have been hidden under our very feet for untold mellennia.
Even with our newfound allies, the Elites, and their valiant and honorable leader, the Arbiter, we are still hopelessly outnumbered. The Prophets set their wills against us--and forces greater than the Covenant wheel malevolently into place, intent upon the destruction of all but the Flood.
As the Master Chief hurtles Earthward in a Forerunner spacecraft, he may be our last chance to stop the Covenant, the Flood, and the ticking time bomb that is the Halo network, before every last soul in the galaxy is destroyed.
