Plot

After fleeing the Covenant's destruction of the human world Reach, the human ship Pillar of Autumn makes a random slipspace jump to avoid leading the Covenant to Earth. Arriving in uncharted space, the crew of the Autumn discover a massive ringworld orbiting a gas giant. When the Covenant attack, Autumn‍ '​s captain, Jacob Keyes, entrusts the ship's AI, Cortana—and her knowledge of defense deployments and the location of Earth—to the supersoldier Master Chief for safekeeping. Master Chief fights off Covenant boarding parties and leaves the Autumn via a lifeboat to the surface of the ringworld while Keyes lands the Autumn on the ring.

On the ringworld, the Chief rallies human survivors and leads a boarding party to rescue Keyes from the Covenant's clutches. Keyes reveals that the Covenant call the ring "Halo", and they believe it is some sort of weapon. The Chief is tasked with finding Halo's control room before the Covenant does. Once Cortana is inserted into the control room, she becomes alarmed and stays behind while she sends Master Chief to find Keyes. While searching for the captain, Master Chief encounters the Flood, a parasitic organism that infects sentient life. The release prompts Halo's caretaker, the AI 343 Guilty Spark, to enlist the Chief's help in activating Halo's defenses. The Chief's activation of the ring from the Control Room is stopped by Cortana, who reveals that Halo's defenses do not kill Flood, but rather their food in an effort to starve them—meaning that activating the ring would wipe out all sentient life in the galaxy. To stop the Flood from spreading and Spark from activating the ring, Cortana devises a plan to detonate the crashed Autumn‍ '​s engines and destroy Halo. Fighting through Flood, Covenant, and Guilty Spark's robotic Sentinels, the Chief manually destabilizes the Autumn‍ '​s reactors and he and Cortana narrowly escape the destruction of the ring via a fighter.

(FROM WIKIPEDIA)

 

Gameplay(SINGLE-PLAYER):