Halo: Combat Evolved
The game opens as the Pillar of Autumn exits slipspace near a mysterious ring-shaped space station, called "Halo" by the enemy in the game, the Covenant. A Covenant fleet attacks and heavily damages the Pillar of Autumn. Captain Keyes initiates "The Cole Protocol," a procedure which destroys all navigation data and is designed to prevent the Covenant from learning the location of Earth. Master Chief and Cortana fight their way to an escape pod, where they eject from the ship along with a group of Marines. As they fly away they can see Keyes attempting to crash land Pillar on Halo.
Master Chief and Cortana are the only survivors of their pod's crash landing, but they soon contact other survivors and help collect them into a fighting force. The survivors tell them that Keyes survived the Pillar of Autumn's crash landing, but has been captured by the Covenant and is being held aboard the Covenant ship Truth and Reconciliation. Master Chief and a force of Marines board the ship and rescue Keyes, who orders Master Chief to beat the Covenant to Halo's control center and to discover Halo's purpose. Master Chief and Cortana travel to a map room called the Silent Cartographer, which leads them to the control room. There, Cortana enters the systems and, discovering something urgent, suddenly sends Master Chief to find Captain Keyes while she stays behind. Keyes was searching for what he believed was a cache of powerful weapons, but when he reaches his position Master Chief finds only signs of a battle and a recording revealing they have accidentally released the Flood, a parasitic alien race capable of spreading itself by infesting other sentient life-forms.
The release of the Flood prompts Halo's AI monitor, 343 Guilty Spark, to recruit Master Chief in retrieving the Index, a device that will activate Halo and prevent the Flood from spreading beyond the facility. After Master Chief retrieves and prepares to use the Index, Cortana re-appears and warns him against the activation. She has discovered that Halo's defense system is a weapon designed to kill all sentient life in the galaxy, which will deny the host bodies the Flood requires to spread, a revelation which Guilty Spark confirms.
Faced with this information, Master Chief and Cortana decide to destroy Halo to prevent its activation. While fighting the Flood, the Covenant, and Guilty Spark's Sentinels, Cortana discovers that the best way to accomplish this is to cause the crashed Pillar of Autumn to self-destruct. However, Captain Keyes' authorization is required to destroy the Autumn, forcing the Chief and Cortana to return to the now-Flood-infested Truth and Reconciliation to search for him. By the time they reach Keyes, however, he has been infected beyond the point of no return by the Flood. Master Chief retrieves Keyes' neural implants directly from his brain and retreats to the Autumn, where Cortana activates the ship's self-destruct sequence. However, 343 Guilty Spark reappears and deactivates the countdown, discovering the record of human history in the process. Master Chief manually destabilizes the Pillar of Autumn's fusion reactors, and he and Cortana narrowly escape the destruction of the ring via a fighter. Cortana scans nearby space and finds that they are apparently the only survivors. Cortana justifies the sacrifices of the other humans on Halo, stating that they had no choice due to the threat posed by Halo, the Covenant and the Flood. Cortana believes it's over but the Master Chief tells her that they're just getting started.
Halo 2
Taking place shortly after the events of the novel Halo: First Strike, Halo 2 opens with the trial of Thel 'Vadam, a Covenant Elite commander aboard the Covenant's mobile capital city of High Charity. The Elite is stripped of his rank, branded a heretic for failing to stop the humans from destroying Halo, and is tortured by Tartarus, the Chieftain of the Covenant Brutes. On Earth, the Master Chief and Sergeant Avery Johnson are commended for their actions at Halo. Lord Terrence Hood awards the soldiers alongside Commander Miranda Keyes, who accepts a medal on behalf of her deceased father, Captain Jacob Keyes.
A Covenant fleet appears outside Earth's defensive perimeter and begins an invasion of the planet. While the UNSC repels most of the fleet, a single Covenant cruiser carrying an important member of the Covenant hierarchy, the High Prophet of Regret, assaults the city of New Mombasa, Kenya. The Master Chief assists in clearing the city of Covenant; with his fleet destroyed, Regret makes a hasty slipspace jump, and Keyes, Johnson, Cortana and the Master Chief follow aboard the UNSC ship In Amber Clad. The crew discover another Halo installation; realizing the danger the ring presents, Keyes sends the Master Chief to kill Regret while she and Johnson find Halo's key to activation, the Index.
Meanwhile, the disgraced Covenant commander is presented before the Prophet Hierarchs, who acknowledge that though the destruction of Halo was his fault, he is no heretic. They offer him the honored position of Arbiter so that he can continue to fight for the Covenant. On his first mission to kill a heretic, the Arbiter discovers 343 Guilty Spark, who the Covenant calls an "oracle," and brings him back to High Charity. Responding to Regret's distress call, High Charity and the Covenant fleet arrive at the new Halo, Installation 05, just before the Master Chief kills Regret. Bombarded from space, the Chief falls into a lake and is rescued by a mysterious tentacled creature.
Regret's death triggers discord among the races of the Covenant, as the Hierarchs have given the Brutes the Elites' traditional job of protecting them in the wake of the death. The Arbiter is sent to find Halo's Index and captures it, Johnson, and Keyes before being confronted by Tartarus. He reveals to the Arbiter that the Prophets have ordered the annihilation of the Elites, and sends the Arbiter falling down a deep chasm.
The Arbiter is saved by the tentacled creature and meets the Master Chief in the bowels of the installation. The creature, Gravemind, is the leader of the Flood on Installation 05. The Gravemind reveals to the Arbiter that the Great Journey would destroy Flood, humans, and Covenant altogether, and sends both the Arbiter and Master Chief to different places to stop Halo's activation. The Master Chief is teleported into High Charity, where a civil war has broken out among the Covenant; In Amber Clad crashes into the city, and Cortana realizes that Gravemind used them as a distraction to infest In Amber Clad and spread the Flood. As the parasite overruns the city, consuming the Prophet of Mercy in the process, the Prophet of Truth orders Tartarus to take Keyes, Johnson, and Guilty Spark to Halo's control room and activate the ring. The Master Chief follows Truth aboard a Forerunner ship leaving the city; Cortana remains behind to destroy High Charity and Halo if Tartarus succeeds in activating the ring.
The Arbiter is sent to the surface of Halo, where he rallies his allies to assault the Brute's position. With the help of Johnson, he confronts Tartarus in Halo's control room. When the Arbiter tries to convince Tartarus that the Prophets have betrayed them both, Tartarus angrily activates the ring, and a battle ensues. The Arbiter and Johnson manage to kill Tartarus while Keyes removes the Index. Instead of shutting down the ring entirely, a system wide fail-safe protocol is triggered, putting Installation 05 and all the other Halo rings on standby for activation from a remote location, which Guilty Spark refers to as "the Ark". As Truth's ship arrives amidst a raging battle on Earth, Hood asks the Master Chief what he is doing aboard the ship. The Chief replies, "Sir, finishing this fight."
In a post-credits scene, Gravemind is seen arriving on High Charity, where Cortana agrees to answer the Flood intelligence's questions
Halo 3
After the events of the comic tie-in Halo: Uprising, the Master Chief crashes in eastern Africa, where he is found by Johnson and the Arbiter. The Chief, Johnson, and company fight Covenant in the jungle and arrive at a UNSC outpost. Here, Keyes and Lord Hood plan a final 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 clears anti-air Covenant defenses so Hood can lead the last of Earth's ships against the Prophet, but Truth activates the buried artifact, creating a slipspace portal which he and his followers enter. A Flood-infested ship crash-lands nearby; Elite forces arrive and vitrify Flood-infected areas of Earth, stopping the threat. Following a message Cortana left aboard the Flood ship, the Chief, Arbiter, Elites, Johnson, Keyes and their troops follow Truth through the portal. Joining them is 343 Guilty Spark, who aids the Chief as he has no function to fulfill after the destruction of his ringworld.
Traveling through the portal, the humans and Elites discover an immense artificial structure known as the Ark, far beyond the edges of the Milky Way galaxy. Here, Truth can remotely activate all the Halos. The Flood arrive aboard High Charity in full force, beginning to infest the installation. Truth captures Johnson, as he needs a human to use Forerunner technology. Keyes is killed attempting a rescue, and Johnson is forced to activate the rings. Gravemind forges a truce with the Chief and Arbiter to stop Truth. The Arbiter, Master Chief, and Flood forces arrive and overwhelm Truth's guards, rescuing Johnson and halting the installations' activation. After the Arbiter kills Truth, Gravemind turns on the Chief and Arbiter.
The Chief, Arbiter and Guilty Spark discover that the Ark is constructing another Halo to replace the one that the Chief previously destroyed. The Chief decides to activate this Halo; the ringworld would eliminate the Flood infestation on the Ark while sparing the galaxy at large from destruction. To activate the ring, the Chief rescues Cortana, who has the Activation Index of the destroyed Halo, from High Charity and destroys the city. Arriving on the new Halo, Cortana warns that Gravemind is trying to rebuild itself on the ring. The Chief, Arbiter, and Johnson travel to Halo's control room to activate the ring. 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 fatally wounds him to protect "his" ring. The Chief destroys Guilty Spark, activates the ring, and escapes the ring's self-destruction on the UNSC frigate Forward Unto Dawn. However, the force of Halo's blast causes the slipspace portal to collapse, resulting in only the front half of Forward Unto Dawn, carrying the Arbiter, making it back to Earth.
A memorial service is held on Earth for the fallen heroes of the Human-Covenant war, during which the Arbiter and Lord Hood briefly exchange words regarding the fallen Master Chief. After the memorial service, the Arbiter and his Elite brethren depart for their home planet, Sanghelios. Meanwhile, the rear half of the Forward Unto Dawn drifts in unknown space. Cortana drops a distress beacon, but acknowledges it may be many years before they are rescued. The Chief enters cryonic sleep, telling Cortana to "wake me, when you need me." If the game is completed at the Legendary difficulty level, the scene continues to show the piece of Forward Unto Dawn drifting towards an unknown planet, revealed in the sequel to be known as "Requiem", the primary setting of the campaign of Halo 4.
Halo Wars
The Spirit of Fire is sent to the ruined planet Harvest to investigate Covenant activity, where Cutter learns that the Covenant has excavated something at the planet's northern pole. When the UNSC's main outpost on Harvest is captured, Cutter orders Forge to retake it. Soon after, Forge scouts the Covenant excavation and discovers that they, under the direction of the Arbiter, have discovered a Forerunner facility. Forge's troops defeat the Covenant forces before they can destroy the installation, and Anders arrives. She determines that the facility is an interstellar map, and recognizes a set of coordinates that points to the human colony of Arcadia.
After repelling a Covenant counterattack on the Forerunner facility, the Spirit of Fire travels to Arcadia, where the Covenant has begun raiding local cities and slaughtering civilians. Forge contacts the local Spartan special forces and assists with the evacuation process. The Covenant builds a giant energy shield to hide the construction of a gigantic Scarab super-weapon, but the UNSC forces use experimental equipment to break through. The Scarab is destroyed, but the Arbiter kidnaps Anders and escapes the planet.
Forge and the Spartans return to the Spirit of Fire and the crew follows Anders' signal to an uncharted planet in another star system. The planet's surface is infested by the parasitic Flood, who attack and assimilate any sentient life they encounter. The Spirit of Fire inadvertently activates a Forerunner docking station and enters the planet's interior. Flood board the ship, and very soon Forerunner Sentinels appear to cleanse the Flood threat and also to destroy all UNSC forces in the process. The crew discovers the planet is actually hollow, with a habitable interior and a miniature sun. The Covenant's plan is to activate a dormant fleet of highly advanced Forerunner starships inside the planet, and use them to obliterate humanity.
While the Forerunner ships are being activated, Anders escapes through a teleportation device and is rescued. Cutter decides to destroy the Forerunner fleet rather than allow the Covenant to use it. Anders formulates a plan to detonate the ship's faster-than-light drive in the planet's sun, as the explosion would cause a supernova. Before they can prepare the reactor, Forge and the Spartans are ambushed by the Arbiter and his Elites. The Spartans dispatch the aliens and Forge kills the Arbiter. The reactor is damaged during the fight, necessitating a manual detonation. Forge volunteers for the suicidal task, telling the Spartans that they will be needed in the coming fight. The Spirit of Fire escapes just as Forge overloads the reactor, destroying the Forerunner fleet. Without its faster-than-light drive, the Spirit of Fire is left drifting in space. The crew enters cryonic sleep for long-term storage as Cutter takes a last look at Forge's empty cryonic tube. If the game is completed on the "Legendary" difficulty mode, Serina wakes Cutter and tells him that "something has happened".
Halo 3: ODST
The game begins with Dutch , Romeo, and Mickey discussing the Covenant ship above New Mombasa, with the Rookie asleep nearby. Buck arrives and introduces Dare. Romeo wakes the Rookie and the team enter their HEVs and drop through the atmosphere; at the last minute, Dare changes their trajectory to miss the ship. The Covenant ship enters slipspace, sending a shockwave toward the ODSTs; the Rookie's pod collides with Mickey's and crashes into a building on the ground, knocking him unconscious for six hours. He awakens and proceeds to find clues as to what happened to his squadmates.
Buck makes a rough landing after the drop and fights through Covenant forces to find Dare. He finds Romeo instead, and the two resolve to find the others and get out of the city. Dutch drops near a nature preserve and helps Marines fighting there. Mickey commandeers a tank and fights his way along a Mombasa boulevard. Meeting up with Dutch, the two defend an ONI base from the Covenant, destroying the facility to keep it from being captured. They are evacuated by a transport and make contact with Buck, arranging a rendezvous at police headquarters, but are shot down. Buck and Romeo rescue Dutch and Mickey, but Romeo is seriously wounded. The squad hijacks a Covenant dropship, but instead of leaving the city, Buck decides to have them turn back and find Dare.
Back in the city, the Rookie is assisted by Virgil (the Superintendent), which leads him to Dare's position. The two reach the Superintendent's data core, which possesses information on something the Covenant is looking for underneath the city. Inside the core, they discover a Covenant Engineer. Dare explains that the Engineers are "biological supercomputers" that have been enslaved by the Covenant, and the one they found wishes to defect to the humans; with the Engineer's information on the Covenant combined with the Superintendent's data, Dare's mission changes from downloading the Superintendent's data to escorting the alien to safety. The Rookie, Dare, and the Engineer reunite with Buck and fight their way out of the city. As they fly away in the transport, the squad watches as Covenant ships destroy New Mombasa.
In the epilogue, one month after the events of the Campaign, the ODST squad has been keeping guard over the Engineer. Sergeant Major Avery Johnson arrives, informing the Engineer that he intends to ask it everything it knows about the Covenant—and what they are looking for—setting the stage for Halo 3. If the campaign is completed on the Legendary difficulty level, a scene shows the Prophet of Truth overseeing the excavation of a Forerunner artifact buried beneath the Superintendent's data core.
Halo Reach
The game opens with the planet Reach in ruins, then flashes back to before the devastating invasion by the Covenant. Noble Team, dispatched to investigate why a communications relay has gone offline, discovers Covenant forces on Reach. Soon after, the team defends "Sword Base", an Office of Naval Intelligence (ONI) installation, from a Covenant vessel. The team meets Catherine Halsey, a scientist and the mastermind behind the Spartan program and their MJOLNIR powered armor. Halsey informs Noble Team that the Covenant forces at the relay were searching for important information.
Jun and Six are dispatched on a covert mission to assess the Covenant's strength and discover an invasion force gathering on the planet. The following morning, Noble Team assists UNSC forces in assaulting a Covenant ground base. When a massive Covenant super-carrier joins the fight, Jorge and Six take part in a plan to destroy the super-carrier using a makeshift bomb. The Spartans use starfighters to infiltrate a smaller Covenant corvette, prepare the bomb and set the corvette on a docking course with the carrier, but the bomb's timer is damaged. Left with no choice, Jorge stays behind and sacrifices himself to destroy the super-carrier. Moments later, huge numbers of Covenant ships arrive at Reach and begin a full-scale invasion.
Six returns to the surface and travels to the city of New Alexandria. The Spartan aids the local military in fighting the Covenant and evacuating the city, reuniting with Noble Team along the way. They retreat to an underground bunker when the Covenant begin to bombard the city with plasma, but Kat is killed by a Covenant sniper before they reach it. Recalled to Sword Base, Noble Team is guided underground to an ancient artifact that Halsey believes is key to winning the war against the Covenant. Six, Carter and Emile are entrusted with transporting the artificial intelligence Cortana, and the information she carries concerning the artifact, to the UNSC ship Pillar of Autumn. Jun leaves the team to escort Halsey to another base.
En route to the Autumn's dry dock, Carter is critically wounded. He rams his ship into a Covenant mobile assault platform, allowing Six and Emile to safely reach the shipyard. Emile uses a mass driver emplacement to defend the Autumn while Six fights through Covenant ground forces to get Cortana to the Autumn's captain, Jacob Keyes. When Emile is slain by Elites, Six remains behind to control the gun, ensuring the Autumn's escape. The Autumn flees from Reach and discovers a Halo ringworld, leading directly to the events of Halo: Combat Evolved.
The post-credits scene puts the player in control of Six's last stand against overwhelming Covenant forces. After sustaining heavy damage, Six drops his or her shattered helmet and is overwhelmed and killed. Decades later, Six's helmet remains on the grassy plains of a now-restored Reach. A narration by Halsey eulogizes Noble Team, who ultimately enabled humanity's victory over the Covenant.
Halo 4
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 Prometheans, 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 attacksInfinity 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 digital versions 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 corridor 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
Halo 5: Guardians
Halo 5: Guardians takes place in the year 2558, and is set eight months after the events of Halo 4. The game follows the human fireteams Blue Team and Fireteam Osiris. Blue Team is led by Master Chief Petty Officer John-117 and is composed of the Spartan-II teammates he fought alongside in the timeframe before Halo: Combat Evolved: Linda-058, an elite sniper; Kelly-087, a scout known for her incredible speed and John's closest friend; and Frederic-104, who is technically Blue Team's highest-ranking officer, but defers to Master Chief. The members of Blue Team are all Spartan-IIs, augmented super soldiers who were abducted and trained as children; they are among the last Spartan-IIs left alive. Fireteam Osiris comprises Spartan-IVs, a newer generation of Spartans who joined the program as adult members of the military. Fireteam Osiris is led by Jameson Locke , a former agent of the Office of Naval Intelligence (ONI) and skilled tracker. The other members of Fireteam Osiris are: Holly Tanaka, a survivor of a Covenant attack on her world; Olympia Vale, who can speak many of the Covenant's dialects; and Edward Buck, a veteran former Orbital Drop Shock Trooper who was previously a main character in Halo 3: ODST.