The Covenant shows no mercy as they continue to assault every human world they can find, but in their way lies humanity's great champion, Spartan-117, the Master Chief. Together with his AI companion Cortana and the last remaining Spartans, the fight continues on two fronts. One takes a crew of Spartans to the charred surface of Reach, the only planet they've ever known as home. But beneath the surface, Dr. Halsey has discovered an ancient secret...one that could alter the course of the war.