The game was originally being developed as a Third-Person Shooter, and would focus on the humans using Guerilla Warfare against the Covenant. On its release, it was changed into a First-Person Shooter, and the Humans technology was matched with the Covenants. The game was later released for Windows and the Macintosh in 2003. On November 15th 2011, Halo: Combat Evolved Anniversary was released to mark the tenth anniversasy of the release of this 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.
The Pillar of Autumn, a Space Ship, exits slipspace nearby space station named the Halo. The Covenant, a race of aliens, severely attack the ship. Captain Keyes begins to destroy all navigation data 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. However, Keyes is needed to authorise the action, leading them to find him on Truth and Reconciliation. However, they find him infected, as well as the ship being infested with the flood. Master Chief retrieves neural implants from his brain, and uses them to activate the self-destruct sequence.
Guilty Spark stops the countdown, forcing Master Chief to manually destabilise the Ships Reactors, leading them to flee on a fighter as the Halo is destroyed.
In a Scene during the Credits, it is revealed that Guilty Spark also escaped the explosion.