In the first game, the Covenant arrive at Reach, one of the last remaining strongholds of the UNSC, and destroy most of the colony.  The Master Chief is able to escape on the ship Pillar of Autumn and the ship flies off into space.  In order to avoid the potentiality of the Covenant from finding Earth, the A.I. Cortana picks a random place to stop the ship; however, when hey stop, they are right in front of the huge ring-world known as Halo.  The Covenant begin to attack the ship, and the Pillar of Autumn is forced to land on Halo.  During the attack on the ship, the Covenant accidentally release a small amount of the Flood which spread rapidly on Halo.  The release of the flood causes the ring's caretaker, 343 Guilty Spark, to convince the Master chief to activate Halo. However, the Master chief learns of Halo's true purpose, which is the destruction of all sentinent life, and chooses to destroy the Pillar of Autumn instead causing the ring to be broken by the explosion.  He escapes with Cortana on a UNSC fighter ship.