Halo 3

Plot

The storyline begins several days after the events of the Halo 2 era. John-117 - the Master Chief, after hitching a ride on Truth's Dreadnought, enters Earth's atmosphere and crash lands in an east African jungle. Sergeant Major Avery Johnson, the Arbiter, and a handful of Marines retrieve him, but was presumed dead until the Master Chief said "Yeah, you're not.". After the short cut scene, the Master Chief sees one of his old enemies, The Arbiter, and runs towards him, pistol in hand, but Johnson holds the Chief back, explaining to him that the Arbiter is their ally in the war now, as they share a common enemy. The group then makes their way through the jungle toward a designated extraction point. Covenant forces are sighted in the area and Johnson makes a strategical decision to split the group up to reduce their chances of getting spotted. Master Chief and the Arbiter continue on their way to the extraction zone with second squad, fighting off numerous Covenant troops in the process, but upon reaching the extraction zone, Avery Johnson's Pelican group consisting of two Pelicans, are ambushed and shot down by two Banshees. Johnson and his men are then captured and taken prisoner by a number of Brutes led by a Gravity Hammer wielding Chieftain. The Master Chief, Arbiter, and company fight their way through hordes of enemies and eventually rescue them, after which a Pelican arrives and picks them up.
The Pelican brings them to Crow's Nest, an underground 21st century facility, believed to be an old NATO base, used as a local base of operations by the UNSC. Here, Commander Miranda Keyes and Lord Hood plan a last-ditch effort to stop the Covenant from activating a Forerunner artefact uncovered outside the ruins of the city of New Mombasa, which is assumed to be The Ark - from where Truth is able to activate all the Halo rings in the galaxy. Their plan is to have the Master Chief with a small group of marines to punch a hole in Truth's anti air defence, so that Lord Hood can initiate a low level strike on the artefact. Their planning is quickly interrupted when power is cut and the Prophet of Truth broadcasts an announcement to all surviving human forces that their doom is inevitable. Soon after that, the Covenant discover the facility and mount a heavy assault. After a brief struggle, the outpost is lost, due to an explosion of a bomb the Chief activated to clear all remaining Covenant forces still inside the base. Master Chief soon receives orders from Commander Keyes to clear a way into the African City of Voi, Kenya via the Tsavo Highway, and destroy all Covenant anti-air defences so that Lord Hood can lead the last of Earth's frigates against Truth and the artefact. Using the opening caused by the ground attack, Hood mounts an all out attack on Truth, but the Prophet activates the artefact and creates an enormous slipspace portal, into which all the Covenant ships retreat. At the same time, a Covenant cruiser controlled by the Flood, a parasitic species, had broken through the Elite's Quarantine of High Charity and arrived suddenly via slipspace and crashes into Voi, and the parasite quickly spreads throughout the entire city.

As the Human forces attempt to fight the infestation, many are infected by the Flood. Soon after the struggle begins Covenant Separatists, a Faction which broke off from the entire Covenant during Halo 2/ Halo 3 (somewhere right after Halo 2 era), lead by Shipmaster Rtas 'Vadum, arrives at Earth and assists the struggle against the infestation, dropping off Orbital Insertion Pods containing allied Elite warriors via the Carrier Shadow of Intent and a few other Separatist cruisers. The Elites inform Commander Keyes that a UNSC construct is aboard the crashed Flood ship, and identify it as Cortana. The Master Chief, Arbiter and the Elites fight off many Flood forms and retrieve Cortana from the Flood ship, losing a few comrades in the making. 343 Guilty Spark (the Elites refer to him as the Oracle) arrives by a Separatist Phantom, and insists that he take Cortana somewhere where he can make repairs, claiming that now his Installation has been destroyed he now must follow his second function of helping the Reclaimers, so decides to help the Master Chief destroy the Flood. Once the Arbiter and the Chief are evacuated, the Elites glass the entire infected half of Africa to prevent the Flood from spreading, which amplifies the hard feelings between Lord Hood and Rtas 'Vadum, Shipmaster of Shadow of Intent, and Fleet Master of the current Covenant Separatist force.

343 Guilty Spark manages to make efficient repairs to Cortana, but the Master Chief realizes it isn't her, but only a recorded message from her. The message states that the Gravemind is coming to Earth on the former Covenent city High Charity and that the portal that Truth retreated into is unknown to the Gravemind, and on the other side is a solution to the Flood infestation, without having to fire the rings. Lord Hood questions Cortana's plan, saying it could be a Flood trap, but the Master Chief firmly tells Hood that he completely trusts her.

Once the Elites and Commander Keyes decide to follow Truth, the remaining Elite and UNSC ships leave Earth via the activated slipspace portal. Lord Hood stays on Earth to fight the remaining infection with some UNSC marines and the rest of the UNSC Army. On the other side of the portal, they find themselves near an immense artificial structure, which is soon revealed to be the real Ark. Arriving at the Ark, they engage a Covenant fleet three times their size, with Rtas 'Vadum stating "Then it is an even fight". After landing on the structure, Guilty Spark helps lead Master Chief and the UNSC armoured force to the Cartographer, or map room of the Ark. After fighting their way in, Chief and the Arbiter find out that the Ark is in fact positioned at the edge of the Milky Way, just beyond the Halo rings' effective firing range. Soon Spark, understanding a Cartographer, explains that the UNSC and the Separatists must deactivate three shield generators, which are inside three separate towers, to access the Control Room of the Ark, where Truth is currently located and plans to activate the rings from.

Upon arrival at the three generators, the Master Chief takes an ODST squad to the first tower, while the Arbiter takes his Elites to the second, and Sergeant Johnson takes another squad to the third. While Master Chief and the Arbiter succeed in deactivating the first and second towers, Johnson radios Keyes, telling her that the enemy forces are too heavy and he must retreat. Miranda Keyes then orders the Arbiter and Master Chief to go and assist him. On arrival to the third tower, Johnson and his men are nowhere to be seen. Master Chief, Arbiter, and the Elites kill the Loyalist force defending the tower and deactivate it. However, just after the final shield barrier protecting the Control Room deactivates, the Flood-infested High Charity arrives via slip-space and crashes onto the Ark, releasing Flood Dispersal Pods as it falls. Spark insists that he must assess the damage that High Charity did to the Ark, so the Arbiter and Elites leave the UNSC to protect Spark from being captured by the Flood. The Chief leads the remaining UNSC and surviving Elite force via vehicles in an attack on the Citadel containing the Control Room, which is defended by numerous Covenant armour forces. After all the forces are defeated, 343 Guilty Spark and the Arbiter return, but then the Arbiter and the Master Chief enter the citadel alone to avoid more casualties. Once inside, they see that Truth has captured Johnson, knowing that only humans can activate Forerunner Installations, but are too far away to save him. Just as Truth is about to force Johnson to activate the rings, Keyes commandeers a Pelican and crashes it in the Control Room, keeping the Brutes at bay with a Shotgun while tending to Johnson. Knowing she can't kill all of the Brutes, she realizes she must kill Johnson and herself to stop the rings from being activated. She is about to shoot Johnson before Truth grabs a Spiker and fires seven shots at her spine with it, killing her. Truth then pushes Johnson's hand down on the Ark's terminal, activating all of the Halos in the galaxy.

Then Master Chief and Arbiter take an elevator up to the Control Room, despite knowing it is too late. As they reach the top, two Flood Tank Forms confront them, but say that the only way to stop the Halos is by joining forces. With the Flood's help, the Master Chief and Arbiter kill all the Covenant forces defending Truth. Upon arrival to the Ark's terminal, they find Johnson mourning over Keyes' dead body, and Truth partially infected. The Arbiter, after a short confrontation with Truth over the truth of the Halos, executes Truth with his Energy Sword, while the Master Chief deactivates all the Halos, and Johnson is seen walking towards the Pelican with the body of Keyes. After the rings are stopped, however, the Gravemind betrays them. Johnson attempts to escape with the Pelican of Commander Keyes, and he escapes, but the Gravemind prevents the Arbiter and Chief from escaping with him.

Johnson radios Chief, telling him to find another way out, so he and the Arbiter shoot their way back to the entrance elevator, but it is deactivated. Behind the elevator is a hole, leading to a lower level of the citadel. Cortana briefly shows herself to the Chief, and leads him to a control panel which reveals her plan - The Ark releases a 'replacement' Halo in the event of one being destroyed. The Chief decides he's going to light the single replacement ring in order to destroy the Flood for good. However, Guilty Spark tells them it will take quite some time to fabricate an Index needed to fire the Halo, but Master Chief remembers that Cortana still has the Index that she acquired on Installation 04. He infiltrates the crashed High Charity, which is completely overrun by Flood. Cortana becomes rampant before the Chief can reach her, but she is able to contain the functional part of herself in a stasis-field, where she waits. Master Chief breaks the stasis-field and retrieves a perfectly functioning Cortana. She tells him to overload High Charity's engines in order to destroy the Gravemind. The Arbiter arrives via a Banshee and helps the Chief fight the Flood. They find a Pelican, which had presumably crashed when the Flood used In Amber Clad to infect High Charity during the Halo 2 era. Damaged but still flyable, the Master Chief uploads Cortana into the Pelican and both him and the Arbiter flee High Charity before it explodes.

Cortana makes contact with Johnson, who safely made it back the Shadow of Intent along with the other human and Elite survivors. He tells her that he will land the frigate Forward Unto Dawn near Halo's Control Room, and will be ready to pick them up and head back to Earth after they activate the Installation. When they arrive on the newly built Halo, they quickly discover that Gravemind is attempting to reconstruct itself on the new ring. The Chief, Arbiter, and Johnson quickly make their way to the Control Room, where they attempt to fire the Halo. However, Guilty Spark realizes that this will destroy his new ring because it is not fully ready yet, as well as the Ark. Guilty Spark becomes rampant, and fires his defensive laser at Johnson as he tries to activate the ring. Guilty Spark soon turns on the Master Chief and locks the Arbiter out of the room. leaving the Spartan to destroy Spark himself. Johnson is critically wounded. He tells the Chief to send him out "with a bang" and dies. Cortana activates Halo, and Chief takes Johnson's Warthog with the Arbiter and race towards the Frigate. With no time to reach the cockpit, the Master Chief uploads Cortana into the frigate to start taking off before the Arbiter is able to take the controls. The front half of the Dawn, containing the Arbiter, makes it through the portal. However, as Halo shakes itself to pieces during it's firing sequence and damages the Ark, the portal deactivates before the other half of the Dawn containing Chief and Cortana made it through.

Back on Earth, Lord Hood holds a memorial service in honour for those who died in the Human-Covenant War, with the Arbiter and many Marines in attendance. With Truth dead, the Covenant destroyed, and the Flood wiped out, the long and devastating war is finally over. Lord Hood thanks the Arbiter for standing by the Master Chief until the end but, he says he can never forgive him for what the Covenant did the to human race, and everyone thinks the Chief is dead. After the service, Shipmaster Rtas 'Vadum, tells the Arbiter that he wants to make sure the Elite homeworld is safe. The Arbiter tells him that with the eradication of the Covenant, they have made it so. He tells the piloting Elites to take them home. Just after this you see the camera zoom in to the memorial where the number 117 is scratched onto the front. Then credits roll.

After the credits, it is revealed that the Master Chief and Cortana are still alive on-board the rear section of the Dawn, which had apparently torn off before going through the portal; the pair are drifting through space. Cortana tells the Chief she will drop a distress beacon, but that it could be years before they are found. The Chief floats to a cryotube, and as he is climbing in Cortana comments "I'll miss you." He replies "Wake me, when you need me."

If the last level is completed on Legendary difficulty, the section of Forward Unto Dawn that Cortana and the Chief are aboard is seen drifting towards an unknown Forerunner planet.