In the sequel to Halo, Spartan-117, better known as the Master Chief, has returned to Earth, where he is promptly needed: despite all precautions taken, the Covenant have discovered the location of humanity's homeworld and are launching their attack. For some reason, however, their fleet is much smaller than expected. To solve this mystery, the Chief must return to the stars, learning more about the mysterious Halo rings and their constructors. Meanwhile, the Covenant Elite who commanded the fleet at the Halo construct is blamed for the destruction of the ring and branded a heretic. The High Prophets grant him another chance, however: he is made the Arbiter, a special operative for the Prophets, ordered to carry out impossible missions. He will be dealing with dissent and rebellion among the Covenant - a conflict where he might find himself to be on the wrong side.