The twenty-sixth century. Once considered clandestine, the Spartan-II program has now gone public. Tales of armored super-soldiers fending off thousands of Covenant attacks have become the stuff of legend. While the Master Chief defends a besieged Earth, and the myriad factions of the Covenant continue their crusade to eliminate humanity, an ultra-secret cell of the Office of Naval Intelligence known as "Section Three" devises a plan to buy the UNSC vital time. It will 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 vessels race to claim it and change the course of the Human-Covenant War. But this reawakened and ancient force may have plans of its own...