Halo 3's story centers on the interstellar war between 26th century humanity, led by the United Nations Space Command, and a collection of alien races known as the Covenant. The player assumes the role of the Master Chief, a cybernetically enhanced supersoldier, as he wages war in defense of humanity, assisted by human Marines as well as an allied alien race called Elites, which is led by the Arbiter. The game features vehicles, weapons, and gameplay not present in previous titles of the series, as well as the addition of saved gameplay films, file sharing, and the Forge map editor; a utility which allows the player to perform modifications to multiplayer levels.
Halo 3 grossed US$300 million in its first week. More than one million people played Halo 3 on Xbox Live in the first twenty hours. As of January 3, 2008, Halo 3 has sold 8.1 million copies, and was the best-selling video game of 2007 in the U.S. Overall, the game was very well-received by critics, with the Forge and multiplayer offerings singled out as strong features. By March 2009 more than 1 billion online matches had been played. A prequel to the game, Halo 3: ODST, was released worldwide on September 22, 2009.
Taking place shortly after the events of the comic mini-series, Halo: Uprising, Halo 3 begins with the Master Chief entering Earth's atmosphere and crashing to the ground in eastern Africa, where he is found by Sgt. Major Johnson and the Arbiter. The Chief, Johnson, and company fight their way out of the jungle and arrive at a UNSC outpost. Here, Commander Keyes and Lord Hood plan a last-ditch effort to stop the Covenant leader, the High Prophet of Truth, from activating a Forerunner artifact uncovered outside the ruins of the city of New Mombasa. The Chief is ordered to clear a way into the city of Voi and to destroy all anti-air Covenant defenses so Hood can lead the last of Earth's ships against the Prophet. Using the opening caused by the ground attack, Hood mounts an offensive against Truth's ship, but the Prophet activates the buried artifact which creates an enormous slipspace portal. As the human ships recover from the shock wave, Truth and his followers enter the portal, while a ship controlled by the Flood crash-lands nearby. Elite forces, allied with humanity, arrive and vitrify Flood-infected areas of Earth, neutralizing the parasitic threat. Following the cryptic message from the human artificial intelligence Cortana left aboard the Flood cruiser, the Chief, Arbiter, Elites, Johnson, Keyes and a handful of Marines follow Truth through the portal. Joining them is the Forerunner construct 343 Guilty Spark, who aids the Chief as he no longer has any function to fulfill after the destruction of his Halo installation in Halo: Combat Evolved.Traveling through the portal, the humans and Elites discover an immense artificial structure, the Ark, far beyond the edges of the Milky Way galaxy. Here Truth can activate all the Halos. The Chief and company activate the installation's map room to find Truth at the Ark's control room. During their journey, the Flood arrive en masse on the former Covenant Holy City High Charity, and begin infesting the installation. While attempting to gain access to The Ark' Control Room, Johnson is captured by Truth; the prophet needs a human to utilize the Forerunner technology. Attempting to rescue Johnson, Keyes is killed by the prophet, and Johnson is forced to activate the rings. The Flood leader Gravemind forges a temporary truce with the Chief and Arbiter in an effort to stop Truth. The Arbiter, Master Chief, and Flood forces arrive and overwhelm Truth's guards, rescuing Johnson and halting the installations' firing. After the Arbiter kills Truth, Gravemind turns on the Chief and Arbiter, who escape the Flood's grasp.
The Chief, Arbiter and Guilty Spark discover that the Ark is creating a new ringworld to replace the one previously destroyed. The Chief decides to activate only this new ring to eliminate the local Flood while sparing the galaxy at large. Before he can activate the ring, however, he needs an Activation Index. Knowing that Cortana acquired a copy of one on the first Halo, he rescues the AI from High Charity and creates a chain reaction to destroy the infested city and severely damage the Gravemind.
Arriving on the new Halo, Cortana warns that the Gravemind is trying to rebuild itself on the ring. The Chief, the Arbiter, and Johnson make their way to the control room, where they will activate Halo's weapon. Guilty Spark explains that because the ring is not yet complete, a premature activation will destroy it and the Ark. When Johnson ignores his warning, Guilty Spark kills him to protect "his" ring. The Chief destroys Guilty Spark, activates the ring, and escapes the ring's blast on a frigate, Forward Unto Dawn.
Only the front half of Forward Unto Dawn, carrying the Arbiter, makes it through the portal. Believing the Chief and Cortana to have perished, a memorial service is held for the fallen heroes of the human and Covenant war. After the memorial service, the Arbiter departs for his home planet, where the Elites are finally free of the Prophets' hegemony. Meanwhile, the rear half of Forward Unto Dawn floats in unknown space. Cortana drops a beacon, but realizes it may be years before they are rescued. The Chief enters cryonic sleep, telling Cortana to "wake me, when you need me." If the level is completed at the Legendary difficulty, the scene continues to show the piece of Forward Unto Dawn drifting towards a mysterious planet.
Versions EditHalo 3 was released in three separate versions. The Standard Edition contains the game disc, manual, and a small poster with the game's control-map and artwork.
The Limited Edition, contained in a metal case, contains the game disc, manual, poster, interactive Xbox 360 bonus disc with several featurettes, and a hard cover bound "Bestiarum", which is a collection of information and art covering the species, cultures, and civilizations of Halo 3.
The final version was marketed as the "Legendary Edition", which contains the game disc, manual, poster, interactive bonus disc, Bestiarum (on one of the DVD discs), Legendary DVD containing special content exclusive to the Legendary Edition, and a scale replica of the Master Chief's helmet as a case for the three discs. The slip-cover packaging unfolds into a large heavy-stock poster of Master Chief. Gamestation stores in the UK also offered a limited edition Master Chief figurine only available to the first 1000 pre-orders.
Upon release, some of the Limited Edition versions of Halo 3 were found to have a defect in the hub that kept the discs in place, which could lead to scratched discs. Microsoft confirmed the problem and offered to replace scratched Halo 3 Limited Edition discs free of charge until the end of January 2008. This was not a problem in either the Legendary Edition or the Standard Edition.