The Mission: The Covenant controls Earth. The Flood is unleashed and on the
loose. With the fate of the galaxy hanging in the balance, Master Chief is the
only one who can save the human race. The entire epic story arc has led up to
this moment. Master Chief, the last of his kind, is a warrior born for combat
and bred for war — and is humanity’s only hope.

The Blockbuster Saga Concludes:

After stowing away on a Forerunner spacecraft, Master Chief, a genetically
altered, cybernetically enhanced Spartan supersoldier, must race back to Earth
to stop the Covenant once and for all and save mankind. With his artificial
intelligence companion Cortana in the grip of the Flood’s intelligent
embodiment, Gravemind, Master Chief and his allies are vastly outnumbered, but
the stakes are too high to do anything but face them head-on.

A Desperate Enemy, A Desperate War:

In “Halo 3,” the third installment of the legendary epic, the war has come to a
climax; the Flood continues to grow in numbers and strength, the armies of Earth
are scattered and decimated, and the Covenant threatens to activate the Halo,
effectively destroying all life in the galaxy. Master Chief’s mission is to stop
the Covenant, destroy the impending Flood threat and, ultimately, save mankind.