The story focuses on John, a Spartan better known as Master Chief, who is awoken from stasis to fight against an alien race known as the Covenant. Humanity are currently on the backfoot, having been at war with the Covenant for many years, recently suffering a crushing defeat at Reach.
They are losing the war, and realise that in order to survive, need to get to the source of the problem, find The Covenant’s homeworld and strike-back. Falling back to their flagship, The Pillar of Autumn, humanity scour the galaxy for a solution. It is this desperation, however, that draws them toward their most important discovery to date, an enormous ring-shaped structure known as Halo.