It begins as the Pillar of Autumn exits slip-space & its crew discovers a large ringworld called Halo. The Covenant pursues the Autumn & attacks. With the ship heavily damaged,The Soldiers led by Master Chief escape along with the ship's AI,Cortana,to prevent the Covenant from discovering the location of Earth.

By the time they got there,Master Chief & Cortana meets with Sergeant Avery Johnson & Miranda Keyes,daughter of the late Captain Jacobs. They discovered that they're being watched by the Elite. Then,the Covenant ambushed them while the Elites open fire. One of the Soldiers gets killed by the Covenant before he gets shot by one of the Elites.

The Elite Commander,Arbiter reveals that the Covenant are the followers of their High Prophet Leaders:Truth,Regret,& Mercy,who are planning to use Halo as a weapon. As they contact more Fleets,The Soldiers are being attacked by the parasites called the Flood & some of them gets turned into one. They are saved by the AI 343 Guilty Spark,who has been Halo's caretaker. The Soldiers gets captured by the Covenant led by the Chieftain,Tartarus. Master Chief & Arbiter manage to reach them. The Flood attacks the Soldiers & the Covenant. Master Chief & Arbiter helped Johnson & Keyes escape as the Gravemind kills Mercy. The two are held captive along with Guilty Spark by Gravemind,who reveals that Arbiter's real name is Thel 'Vadam,who rebelled against the Covenants. Johnson & Keyes fled to the Fleet where Tartarus & the Covenants attack. Master Chief,Arbiter,& Guilty Spark quickly escape & rushed towards the ship. Tartarus kills Keyes & wounded Johnson. Two of the Covenant Ships are about to launch for earth,but Johnson manage to crash one of the fleet before dying. The other ship with Truth inside escapes. Cortana learns of the Ring's whereabouts. Master Chief & Arbiter head towards the Ring while the Soldiers evacuated.

They are confronted by Tartarus,who overpowered Arbiter,but Master Chief disarmed Tartarus & decapitates him with Arbiter's Energy Sword. They later confronts Regret,who tells them that the Covenants will never stop & Arbiter fatally stabs him. Master Chief activates the Rings,causing it to self destruct. As they tried to escape,the force of Halo's blast causes the slipspace portal to collapse,resulting in only the front half of Foward Unto Dawn,carrying the Arbiter,making it back to Earth. The Arbiter informs Lord Hood of the Covenants invasion & the disappearance of Master Chief & Cortana. They must prepare the Soldiers & the Elites to prepare for the attack on the Covenants. In Space,Master Chief was in Cryosleep on the other half of the ship where he is awaken by Cortana.