Closure
=== Setting and characters ===
Halo 4 takes place in a futuristic science fiction setting in the year 2557, four years after the events of Halo 3. Backstory details that hundreds of thousands of years before the modern era, humans were one of several interstellar civilizations. Following a war with the parasitic Flood, the humans came into conflict with the Forerunners, a powerful race that upheld a policy of benevolent shepherding of other races known as the Mantle of Responsibility. Humanity had begun an aggressive campaign of destroying any Flood infestations on a planet, which was misinterpreted as a desire for conquest. After centuries of war the Forerunners defeated the humans, ancestors of the contemporary human race, and stripped them of their technology and empire. The Forerunners soon fought the Flood themselves; after exhausting every other strategic option available to them, the Forerunners activated weapons of mass destruction known as the Halo Array. The Array's firing killed all sentient life in the galaxy to deprive the Flood of their food. Life that the Forerunners cataloged was then reseeded throughout the galaxy.
In the 26th century, space-faring humanity (organized under the United Nations Space Command, or UNSC) comes under attack from a splinter faction of the former Covenant, an alien collective of species that worships the Forerunners as gods. This Covenant faction believes that activating the Halos will bring about salvation, not destruction. The human supersoldier Master Chief John-117, along with his artificial intelligence companion Cortana, are instrumental in stopping the Halos from being activated, and the Flood from once again menacing the galaxy. At the end of Halo 3, Chief and Cortana are left stranded in unknown space aboard the remains of the vessel Forward Unto Dawn. Much of Halo 4's campaign and Spartan Ops' missions take place on or near the Forerunner planet Requiem. Part of the game also takes place on Ivanoff Research Station—a human base in orbit around the Halo ringworld Installation 03—and in Earth's orbit.
=== Campaign ===
Four years after the events of Halo 3, UNSC Forward Unto Dawn drifts towards Requiem from the destroyed ring. Cortana wakes Master Chief from cryonic sleep shortly before rogue post-war Covenant forces board the vessel. The remnants of Forward Unto Dawn are caught in Requiem's gravity well and pass through an opening to crash-land on its interior. As Chief and Cortana explore Requiem, fighting hostile Covenant and mechanical warriors called "Promethean Knights", Cortana malfunctions. She reveals that she is experiencing "rampancy", a declining mental state where AIs past their operating expectancies "think" themselves to death. Chief promises to get Cortana to Earth, believing that Cortana's creator, Doctor Catherine Halsey, can fix Cortana's condition. Chief and Cortana pick up garbled transmissions from a human ship, UNSC Infinity, who have picked up Dawn's distress call. Cortana attempts to warn Infinity away from Requiem's gravity well and directs the Chief to deactivate what she believes are communications jammers. Instead, the Chief unwittingly releases the Didact, an ancient Forerunner warrior, from imprisonment. The Didact takes control of the Prometheans and Covenant, and attacks Infinity after it is dragged into Requiem.
Chief makes contact with Infinity and helps repel the Didact's attack. Chief and Cortana recommend attacking the Didact while he is vulnerable, but Infinity captain Del Rio orders them to destroy the gravity well so their ship can escape. In the process, Chief is contacted by a Forerunner known as the Librarian—the wife of the Didact, as well as ancient humanity's protector. She explains that the Forerunners were divided on how best to combat the Flood. After failing to discover a way to immunize biological beings from the parasite, the Didact used a device called the Composer to convert the warriors under his command into mechanical Promethean Knights immune to infection. Requiring more soldiers, the Didact forcibly converted captured humans into Prometheans, before being stopped and imprisoned by the Librarian. The Librarian, who has guided humanity's development, accelerates the Chief's evolution; this grants him immunity to the Composer.
After destroying the gravity well, Del Rio orders a retreat back to Earth, doubting the Chief and Cortana's testimony. The Master Chief disobeys orders to stand down and relinquish the malfunctioning Cortana, and stays behind to oppose the Didact. Infinity executive officer Commander Lasky gives Chief an armed transport and wishes him luck. Chief and Cortana attempt to sabotage the Didact's ship before he leaves, but when they are unsuccessful they follow the Didact to a Halo ring, Installation 03. The Composer has been moved from the ring onto the nearby Ivanoff Research Station, which the Covenant attack. The Chief defends Ivanoff, but the Didact retrieves the Composer and uses it on the station, composing every individual except the Chief.
Chief and Cortana use a fighter to follow the Didact's ship through slipspace towards Earth. Aided by Infinity, now commanded by Lasky, and the UNSC home fleet, the Chief boards the Didact's ship with a nuclear warhead. Cortana inserts copies of herself into the Didact's computer systems to overwhelm the Didact's shield, but not before the Didact directs the Composer at Earth. With the help of Cortana, the Chief defeats the Didact, who falls into a slipspace portal generated underneath the Composer. The Master Chief activates the bomb but is saved by Cortana, who sacrifices herself. The Chief is found by a UNSC rescue team and is taken back to Infinity, where he, mourning the loss of Cortana, talks with Lasky about Earth, humanity, and what his duty as a soldier means.
In a post-credits cutscene, UNSC forces descend on the city of New Phoenix, the location the Didact used the Composer on, to find its inhabitants composed. In a narration, the Didact proclaims the Forerunner's role as custodians of the galaxy having to bear the Mantle of Responsibility, and humanity as the greatest threat in the galaxy. Master Chief removes his armor aboard Infinity; if the player completes the game on Legendary difficulty, the Chief's eyes are briefly shown.
=== Spartan Ops ===
The events of Spartan Ops take place six months after Halo 4's campaign. Infinity returns to Requiem, where Covenant insurgents—led by Jul 'Mdama, who styles himself as "the Didact's Hand"—and Prometheans are still active. Sarah Palmer, commander of the Spartan-IVs, deploys Spartan squads to clear out the Covenant and Promethean forces in Requiem's interior in order to set up research bases. Crimson Squad recovers a mysterious artifact excavated by Covenant; Infinity scientist Doctor Glassman disappears after studying the device.
Catherine Halsey is brought to Infinity due to her knowledge of Forerunner technology. She is kept under guard by marines and Spartans including Gabriel Thorne, who lost his family during the Didact's attack on New Phoenix. While studying the artifact, Halsey begins receiving messages on her computer from an anonymous sender. The source of the transmissions is 'Mdama, who captures Glassman and forces him to work on a Forerunner device that supposedly contains the Librarian. Pursuing 'Mdama's troops, Crimson Squad recovers another artifact. When Halsey examines the device, it displays images Thorne recognizes as depicting New Phoenix. Lasky reveals to Halsey and the Spartans that the UNSC knows Prometheans are former humans. Approaching the artifact looking for answers, Thorne is teleported to Requiem and captured by Covenant. Palmer arrests Halsey for communicating with 'Mdama. Thorne escapes captivity outside 'Mdama's base, and escapes with Glassman; they are rescued by Majestic Squad.
Halsey uses an override code to force Infinity's AI, Roland, to assist her. Seeking information withheld from her concerning the Librarian, Halsey learns that Master Chief is alive. Halsey contacts M'dama to try and strike a deal, as they both want to find the Librarian; Roland breaks free of Halsey's control and summons guards to apprehend Halsey. Lasky decides to place Halsey in cryogenic sleep to prevent further trouble, but Promethean forces invade Infinity, capture Halsey and teleport her to 'Mdama's base. Serin Osman, head of the Office of Naval Intelligence (ONI), orders Lasky to kill Halsey, but he is reluctant to do so. Palmer sets off to Requiem to kill Halsey; Lasky tells Majestic Squad to intervene and rescue Halsey from 'Mdama.
Halsey steps into the Forerunner device and makes contact with the Librarian. The Librarian gives Halsey the two pieces of the "Janus Key", an instrument that provides the location for all Forerunner technology in the galaxy, and instructs Halsey to use the key to advance humankind. Upon receiving the key, Halsey exits the device and 'Mdama takes half of the key from her. Halsey passes the second piece to Thorne as Majestic Squad and Palmer arrive. Palmer wounds Halsey in the arm just before a Promethean teleports 'Mdama and Halsey from the base. 'Mdama sets Requiem to collide with the nearby sun and his forces evacuate the planet. Crimson and Majestic disable devices that anchor Infinity to Requiem, allowing the ship to escape before the installation is destroyed. Meanwhile, Halsey, who lost her wounded arm, tells 'Mdama she has been betrayed by the UNSC and offers to ally herself to exact revenge.
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