Plot and setting
In the distant past, a powerful race called the Forerunners fought an alien parasite known as the Flood. The Flood, which spread through infestation of sentient life, overran much of the Milky Way Galaxy. One of the races affected was humanity, who came into conflict with the Forerunners as a result. Exhausted by their war and after having explored all other options, the Forerunners conceived a weapon of last resort to combat the Flood, called the Halo Array. Using an installation known as the Ark, they built large ring-shaped megastructures known as Halos. The Halo Array, when activated, would destroy all sentient life in the galaxy – depriving the Flood of their food, after which samples of the lost lifeforms would be reseeded. Delaying as long as they could, the Forerunners activated the rings and disappeared. Tens of thousands of years later, in the 26th century, humanity – under the auspices of the United Nations Space Command, or UNSC – colonize many worlds thanks to the development of faster-than-light "slipspace" travel. Tensions escalate between the older and more stable "Inner Colonies" and the remote "Outer Colonies", leading to civil war. The UNSC sponsors the Spartan-II program to create an elite group of enhanced supersoldiers, whose purpose is to suppress the rebellion covertly. In the year 2525, human worlds come under attack by a theocratic alliance of alien races known as the Covenant. The Covenant leadership declares humanity heretics and an affront to their gods – the Forerunners – and begins a holy war of genocide. The Covenant's superior technology and numbers prove decisive advantages; though effective, the Spartans are too few in number to turn the tide of battle in humanity's favor.
In 2552, the Covenant arrive at the UNSC's last major stronghold beyond Earth – the planet Reach, in orbit of Epsilon Eridani – and launch an invasion of the planet. The last Spartans and UNSC military are unable to stop the Covenant from bombarding the planet. The UNSC Navy suffers devastating losses, and every Spartan but their leader, player character Master Chief John-117, is believed lost in action. Following directives to avoid leading the Covenant to Earth, Cortana, the AI aboard the fleeing UNSC ship Pillar of Autumn, selects random coordinates that lead the ship to a Halo installation. These events form the backdrop to the first game in the series, Halo: Combat Evolved.
Over the course of the game, the Covenant pursue, damaging the Autumn and leading its crew to wage a guerrilla war on the ring's surface. The Covenant accidentally release Flood imprisoned on the ring; to nullify the threat, the ring's AI caretaker, 343 Guilty Spark, enlists the help of the Master Chief in activating Halo. Cortana reveals that Halo's activation would mean their own destruction. Master Chief and Cortana instead detonate Autumn's engines, destroying Halo and preventing the escape of the Flood.
Between the events of the first game and Halo 2, the Chief and a few other survivors return to Earth, warning of an impending Covenant attack; this attack forms the first mission of the game. Meanwhile, the Covenant, unaware of the destructive nature of the rings, attempt to fire another ring, Installation 05, to fulfill their religious prophecy and to do so launch an attack on Earth. Helping the Chief to defend the Earth are Sergeant Avery Johnson and Commander Miranda Keyes. One race in the Covenant, the Elites, learn the truth about the rings and are led by a former Elite commander named the Arbiter (also a player character) to join forces with humanity to stop the firing of the ring. Though they are successful, the unexpected shutdown of the installation triggers a fail-safe protocol, priming all the rings for firing from the Ark.
Halo 3 picks up immediately after, with the Covenant's leader, the High Prophet of Truth, and the remaining loyalist Covenant uncovering a portal to the Ark on Earth. A battle rages on Earth between human forces, the Covenant, and a Flood-infected army. Eventually, the Covenant is repulsed and flees through a slipspace portal, and the Flood is purged from Earth, though much of Africa has been devastated in the battle.
Chief, Arbiter, Elites, Johnson, Keyes, and U.N.S.C. troops follow Truth through the portal, joined by 343 Guilty Spark, who aids the Chief as he has no function to fulfill after the destruction of his ringworld. During the ensuing battle between humans, Covenant, and the Flood on the Ark, Johnson is captured by Truth to activate the Halo rings. Keyes tries to intervene, but is killed. Master Chief and the Arbiter, aided by the Flood, kill the Covenant "Prophet of Truth" and rescue Johnson. After finding out that the Ark is constructing a replacement for Guilty Spark's Halo installation, Master Chief decides to fire the unstable Halo, destroying both the Flood and the Ark. After finding out about their plan, Guilty Spark kills Johnson and tries to kill Master Chief, but Master Chief destroys Guilty Spark and activates the Halo. Master Chief and the Arbiter escape on the UNSC frigate Forward Unto Dawn, but the slipspace portal closes with Master Chief's half trapped in space. After a memorial service on Earth for the victims of the Human-Covenant war, the Arbiter and his Elite brethren depart for their home planet. The rear half of Forward Unto Dawn floats in deep space. Cortana activates a distress beacon and the Chief goes into hibernation.
Four years later, the Chief and Cortana awake to find they are being attacked by a Covenant splinter group (Halo 4). Caught in an artificial gravity well, they are pulled into the artificial Forerunner planet of Requiem, along with the UNSC ship Infinity. Master Chief, fighting Covenant and AI soldiers called Prometheans, unwittingly releases a hostile Forerunner warrior called the Didact. While seeking a way to combat the Didact, Cortana reveals that she is becoming "rampant" (an inevitable state leading to insanity that AIs reach when their knowledge base finally becomes too large). Master Chief is eventually contacted by a Forerunner consciousness called the Librarian (The Didact's Wife), who tells him about the Didact: during the crisis with the Flood, the Didact tried to use a device called the Composer to transform humanity into creatures like the Prometheans. He did this because he needed extra soldiers in the war. His own race then imprisoned him along with Forerunner soldiers and humans he had already converted. The Composer was hidden. After the Didact leaves Requiem and retrieves the Composer, Master Chief pursues him to Earth, with Master Chief and Cortana working together to destroy the Composer. Master Chief succeeds, but Cortana sacrifices herself to save him from the resultant explosion. Master Chief is later found floating among the rubble, and taken back to Infinity.
Halo 5: Guardians begins eight months later. Blue Team — consisting of four of the five remaining Spartans (Master Chief, Fred-104, Kelly-87 and Linda-58), receives signals from Cortana indicating that she has survived her death by taking advantage of Forerunner AI technology. The Chief, disobeying direct orders, travels to the planet Genesis to reunite with her. A team of Spartans from a competing program, Fireteam Osiris, are dispatched to apprehend him. This journey takes them to several places around the galaxy, including the Elite homeworld of Sanghelios, where the Arbiter leads an offensive against the final remnants of the loyalist Covenant. As they travel, Osiris discover that Cortana is activating ancient Forerunner constructs — the eponymous Guardians — with which she plans to impose permanent galactic peace on pain of death. She holds the Master Chief's team captive, and while Fireteam Osiris are able to rescue them, they are not able to stop Cortana from executing her plan, disabling most contemporary technology and bringing all remaining sentient species under her aegis. As the game ends, Blue Team, Fireteam Osiris and the Arbiter regroup to plan their resistance.
- Halo Wars (2009)
- Halo 3: ODST (2009)
- Halo: Reach (2010)
- Halo: Combat Evolved Anniversary (2011)
- Halo: Spartan Assault (2013)
- Halo: The Master Chief Collection (2014)
- Halo: Spartan Strike (2015)
- Halo Wars 2 (2017)
In other media
Microsoft announced at Comic-Con 2009 that it was overseeing production of a series of seven short anime films, together called Halo Legends. Financed by 343 Industries, the animation was created by five Japanese production houses: Bones, Casio Entertainment, Production I.G., Studio 4°C, and Toei Animation. Shinji Aramaki, creator and director of Appleseed and Appleseed Ex Machina, served as the project's creative director. Warner Bros. distributed Legends on DVD and Blu-ray in February 2010. Six of the stories are officially part of the Halo canon, with the seventh, made by Toei, intended to be a parody of the universe. In mid-July 2015, 343 Industries announced that a new animated series of Halo will be included in the Halo 5: Guardians Limited Edition and Collector's Edition called Halo: The Fall of Reach, and will be based on the book Halo: The Fall of Reach by Eric Nylund.