The gameplay is preserved as it was from the original game. The game is a First-Person Shooter, where the player must shoot enemies and complete levels. The player can use a vast array of weapons (Though only two can be carried at once), pilot vehicles and use a Motion Tracker HUD to assist with finding enemies within a certain range, providing they are moving
After the Covenant destroys Reach, The Pillar of Autumn, a Space Ship, exits slipspace to avoid leading the Covenant to Earth, and ends up nearby a space station named the Halo. The Covenant, a race of aliens, severely attack the ship. The Captain of the ship, Jacob Keyes begins to destroy all navigation data (But not before entrusting it to Cortana) to prevent the Covenant from finding earth, while Master Chief (The Protagonist) and Cortana (Master Chief's Accomplice) fight their way to an escape pod, where they escape the ship, while Captain Keyes attempts to crash land the doomed ship into the Halo.
While gathering survivors from the attack, they learn that Keyes survived the landing, but was captured by the Covenant. Master Chief boards the Covenant Ship named Truth and Reconciliation to rescue Keyes. Keyes tells them to get to the Halo's control center and find out its purpose. Master Chief and Cortana find the Silent Cartographer, which leads them to the Control Room. Cortana enters the systems, finds something urgent, and urges Master Chief to get Captain Keyes. They find that they have accidently released The Flood, a Parasitic Alien Race that infests life-forms to spread.
The AI Monitor of the Halo, 343 Guilty Spark, tells Master Chief to retrieve the Index, which will Activate the Halo and stop the Flood. As Master Chief prepares to activate the Halo, Cortana warns him that the Halo is a weapon designed to kill all life in the Galaxy, which Guilty Spark confirms as true, and that it will deny the Flood any more hosts, stopping its spread.
Rather than allow this to happen, Master Chief and Cortana decide to destroy the Halo in order to prevent it from ever being activated. They fight the Covenant, the Flood and Sentinels of Guilty Spark to find the Pillar of Autumn in order to cause it to self-destruct. Master Chief destabilises the Ships Reactors, leading them to flee as the Halo is destroyed.