In the distant past, an intellectual 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 went into space and came into conflict with the Forerunners. Exhausted by their war and after having explored all other options, the Forerunners conceived a weapon of last resort to combat the Flood. 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.[12] 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 creates an elite group of enhanced supersoldiers, known as Spartans, to suppress the rebellion covertly.[13] In the year 2525, the human colony of Harvest falls 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 Reach, the UNSC's last major stronghold beyond Earth, and launch an invasion of the planet. The last Spartans and UNSC military are unable to stop the Covenant from bombarding the surface with plasma. 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. The Covenant follow, damaging the Autumn and leading its crew to take the fight to the ring's surface. The Covenant accidentally release Flood imprisoned on the ring; in order to nullify the threat, the ring's AI caretaker, 343 Guilty Spark, enlists the help of the Spartan, John-117, or Master Chief. However, before the pair can activate Halo's defenses, Cortana reveals that Halo's activation would mean their own destruction. Master Chief and Cortana instead detonate the Autumn's engines, destroying Halo and preventing the escape of the Flood. They return to Earth with a few other human survivors, warning of an impending Covenant attack. The Covenant, unaware of the destructive nature of the rings, attempt to fire another ring, Installation 05, in order to fulfill their religious prophecy.[14] 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 join forces with humanity in order 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 one location, referred to as the Ark.[15] Still oblivious to the true nature of the rings, the CovenantHigh Prophet of Truth and the remaining loyalist Covenant proceed to head to Earth, where they believe the Ark is buried. 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, resulting in the devastation of much of Africa.
Following a message Cortana left aboard the Flood ship, the Chief, Arbiter, Elites, Johnson, Keyes, and their 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, himself infected by Flood, to activate the Halo rings. Keyes tries to intervene, but is killed by Truth. Master Chief and the Arbiter, aided by the Flood, take down 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, and manages to rescue Cortana to use the Activation Index she contains. 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 attempt to escape on the UNSC frigate Forward Unto Dawn, but the slipspace portal, impacted by the Halo's blastwave, shuts 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. But it is shown that the rear half of Forward Unto Dawn is floating in space, with the Chief and Cortana on board. 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. Caught in an artificial gravity well, they are pulled down, with the remains of the Forward Unto Dawn, onto the artificial Forerunner planet of Requiem, along with the UNSC shipInfinity.[16][17] Master Chief, fighting both 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, a state in AIs when their knowledge base becomes too large and they gradually go insane. Master Chief is eventually contacted by a Forerunner consciousness called the Librarian, 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 was imprisoned by his own race, with Forerunner soldiers and humans he had already converted as his guardians, and the Composer 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.
