• The Master Chief entering Earth's atmosphere and smashing to the ground around the west part of Africa, where he is found by Sergeant Major Avery Johnson and the Arbiter. The Chief is only just prevented, by Seargant Johnson, from killing the Arbiter because he is an elite. The Chief, Johnson, and company fight their way to a UNSC outpost. Here, Commander Keyes and Lord Hood plan a last-ditch effort to stop the Covenant leader, the High Prophet of Truth, from activating a Forerunner (an extinct, highly advanced race) artifact uncovered outside the ruins of the city of New Mombasa. The Chief is ordered to clear a way into the city of Voi and to destroy all anti-air Covenant defenses so Hood can lead the last of Earth's ships against the Prophet. Using the opening caused by the ground attack, Hood mounts an offensive against Truth's ship, but the Prophet activates the buried artifact and creates an enormous, stable slipspace portal he and his followers enter. As the human ships recover from the shock wave, a ship controlled by the Flood, a parasitic race that destroyed the Forerunners, arrives via slipspace and crash-lands nearby.Covenant Elite forces, allied with humanity, arrive and vitrify Flood-infected areas of Earth, neutralizing the parasitic threat. Following the cryptic message from the human A.I. construct Cortana left aboard the Flood cruiser, the Master Chief, Arbiter, Elites, Johnson, Keyes and a handful of marines follow Truth through the portal. Joining them is the Forerunner construct 343 Guilty Spark, who aids the Master Chief because his ringworld was destroyed in Halo 1.

    Traveling through the portal, the humans and Elites discover an immense artificial structure—the Ark—which is well outside the Milky Way galaxy. Here, the Prophet can activate all the Halos. The Chief and company quickly activate the installation's Cartographer to find Truth; in the process, Guilty Spark discovers that the Ark is creating a new ringworld to replace his destroyed installation. Guided by Guilty Spark, the Chief and the Arbiter make their way toward the Ark's control room. During their journey, the Flood arrive on the former Covenant Holy City High Charity via slipspace, and they begin infesting the installation.Johnson is captured by Truth to activate the Ark because only a human can utilize the Forerunner technology. The Flood-controlling intelligence Gravemind forges a truce with the Chief and Arbiter in an effort to stop Truth. Soon after the Prophet kills Keyes, the installation's firing is halted. After the Arbiter kills Truth, Gravemind turns on the Chief and Arbiter. Master Chief decides to activate the new ring (a replica of the destroyed Installation 04) being built at the Ark, sparing the galaxy at large while eliminating the local Flood. Before he can activate Halo, he needs an Activation Index, which Cortana acquired in Halo.The Chief saves Cortana from High Charity and overloads its main reactor to destroy the infested "versatile" city and Gravemind with it.

    Arriving on the new Halo, Cortana warns that the Gravemind is trying to rebuild itself on the ring. The Chief, the Arbiter, and Johnson make their way to the control room, where they will activate Halo's weapon. 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 kills him to protect "his" ring.Master Chief destroys Guilty Spark, activates the ring, and escapes with the Arbiter and Cortana to Forward Unto Dawn, a UNSC ship, as Halo fires.

    Back on Earth, the front half of Forward Unto Dawn crashes into the ocean, and workers cut the Arbiter out of the wreckage. A memorial service is held for the fallen heroes of the human and Covenant war; the Master Chief appears to have perished as well. After the memorial service, the Arbiter departs for his home planet, where the Elites are finally free of the Prophets' hegemony.Masterchief and Cortana did survive,they were both floating in space.....the ship was then drifting to an unidentified planet.......