The Spartan-II program has gone public. Tales of super-soldiers fending off thousands of Covenant attacks have become the stuff of legend. But just how many Spartans are left? While the Master Chief defends a besieged Earth, and the myriad factions of the Covenant continue their crusade to eliminate humanity, an ultrasecret cell of the Office of Naval Intelligence known as Section Three devises a plan to buy the UNSC vital time. They're going to need hundreds of willing soldiers, though ...and one more Spartan to get the job done. The planet Onyx is virtually abandoned and the perfect place to set this new plan in motion. But when the Master Chief destroys Halo, something is triggered deep within Onyx: Ancient Forerunner technology stirs, and fleets of UNSC and Covenant race to claim it to change the course of the Human-Covenant War. But this reawakened and ancient force may have plans of its own ...

摧毁光晕后，从光晕上成功出逃的约翰-117，尽力收拢被打散的人类残余部队，力图回到地球。 而致远星也并未被圣约人彻底摧毁，约翰-117的部下大多还活着。这是因为致远星上有着圣约人渴望得到的光晕建造者所留下的“圣物”。 约翰-117超人一般永不停息地战斗着，居然成功抢夺了一艘敌舰杀回致远星，救下了自己那些已夺得“圣物”的部下。然而在逃回地球的过程中，他们发现圣约人竟集结了约五百艘战舰的庞大舰队，意欲进攻地球！地球的命运已经危如累卵……

人类进入宇宙殖民时代后，与强大而好战的圣约人遭遇，圣约人宣称人类亵渎了他们的信仰，对人类发动了全面战争。 很早以前，人类就在进行一个绝密军事计划——“斯巴达II”。该计划培育了数十名基因强化战士，这些超级生化士兵装备着配有人工智能的能量盔甲，战斗力之强超出想像。战争爆发后，以约翰为首的超级战士们被投入战场，果然所向披靡。 然而这支小小的“超人”特种部队毕竟无法挽回人类在太空战场