The Covenant return to Earth.
Sent to retrieve what they believed would be the corpse of the Master Chief, Sergeant Avery Johnson and the renegade Elite known as the Arbiter locate the Spartan's body in Eastern Africa. Despite having fallen from the Forerunner dreadnought as it passed through the Earth's upper-atmosphere, the Master Chief has somehow survived. Part of him, however, is still missing.
Cortana, the onboard artificial intelligence construct who had guided his journey thus far remains on the other side of the galaxy, in the Flood-infested confines of the Covenant city of High Charity.
Shortly after recovering SPARTAN-117, Commander Miranda Keyes reveals that the Covenant have unearthed a massive Forerunner artifact near the town of Voi. The Master Chief and the Arbiter push on to meet them, but by the time they arrive, they are too late.
Using the Forerunner dreadnought, the Prophet of Truth activates the artifact and reveals its secret. In the sky above the ancient machinery was a Portal, the source of the Covenant's focus. With evidence suggesting that Cortana might have a solution waiting for them on the other side, the Master Chief, the Arbiter, and all of the forces under Commander Keyes push into the mysterious gateway and arrive high above an immense Forerunner structure, Installation 00—the Ark.