Master Chief Petty Officer Spartan John-117 arrives back on Earth by slowing away on the Forerunner Dreadnought that left High Charity in the closing events of Halo 2. He meets up with the Arbiter Thel 'Vadam and Sergeant Johnson and proceeds to reconnoiter with UNSC forces. Admiral Hood and Commander Miranda Keyes devise a plan to attack the Portal at Voi which the Covenant were trying to activate. John-117 successfully breaches the Covenant anti-air defenses, allowing the UNSC forces in the area to attack the portal directly, but they are ineffective. The portal is activated, and the Covenant forces use it to travel to the Ark.
Before another plan can be devised, a Covenant ship infected by the Flood arrives at Voi, and the UNSC and forces under Thel Vadam are forced to contain the threat. In the process John finds a message from Cortana declaring a threat to Earth from the Flood Gravemind aboard High Charity. He decides to travel with Commander Miranda Keyes aboard the UNSC Forward Unto Dawn and Thel 'Vadam's fleet through the portal to the Ark to stop the Flood and stop Truth from activating the Halo array.
On the Ark, the UNSC allied forces find Truth trying to activate the array, and a new Halo being constructed in the Ark's forge. They decide to fire the unfinished Halo to destroy the Flood and all of Truth's forces on the Ark, turning 343 Guilty Spark into an enemy again. John manages to recover Cortana and fire the Halo, destroying the Flood, but also leaving him adrift aboard the rear half of the now derelict Forward Unto Dawn.
A sampling of the Halo 3 soundtrack as provided by Spotify.
John-117 continues the fight with the Covenant on Earth, but follows the Prophet of Truth to the Lesser Ark to stop the firing of the Halo Array.
Halo 3 finishes John-117’s fight against the Covenant and the Flood, with the help of many, including Arbiter Thel ‘Vadam, Sergeant Johnson, Miranda Keyes, and 343 Guilty Spark.
Leader, Blue Team
Chief of Naval Operations, UNSC
Commander, UNSC Forward Unto Dawn
UNSC AI for John-117
Commander, Swords of Sanghelios
Sergeant Major, UNSC
Monitor, Installation 04
Central Intelligence, The Flood
Jiralhanae Commander, Covenant
Forerunner Contender-class Ancilla
When the Prophet of Truth uses the Portal at Voi to travel to the Ark, the UNSC forces give chase in the UNSC Forward unto Dawn and the Shadow of Intent.