2526. It has been more than a year since humanity first encountered the hostile military alliance of alien races known as the Covenant, and several weeks after the United Nations Space Command's devastating counterattack of Operation: SILENT STORM was deemed an overwhelming success. The UNSC has put its faith in the hands of the Spartans, led by the legendary Master Chief, John-117: enhanced super-soldiers raised and trained from childhood via a clandestine black ops project to be living weapons.

But the Covenant-enraged and fearful of their enemy's unexpected strategies and prowess-is not taking its recent defeat lightly, and is now fully determined to eradicate humanity from existence, brutally overrunning the ill-fated planets of the Outer Colonies faster than retreats can be ordered.