Master Chief is the last super soldier who is able to fight on the Pillar of Autumn. The Pillar of Autumn jumped (teleported) from the planet Reach. Reach was being destroyed, so they escaped. The ship is piloted by Captain Keyes. The ship arrives in front of Halo. Halo is surrounded by many Covenant ships. The Covenant board the Pillar of Autumn to kill the humans. Keyes orders the Master Chief (who is sleeping while frozen) to be awakened. After putting Cortana (an Artificial Intelligence) in his head, the Master Chief escapes in a lifepod with other Marines. He did this on order by Captain Keyes, who knew they could not win. Captain Keyes decides to crash his ship onto Halo, before it is completely destroyed by the Covenant. The Master Chief rescues as many Marines as he can. Cortana discovers Captain Keyes is still alive. Master Chief goes to rescue him from a Covenant Ship. They escape with the Captain and more Marines. They find a map-room to find the control room for Halo. They want to stop the Covenant from using Halo, because it is a weapon. The Master Chief fights through many aliens to the Control Room. He puts Cortana into the data. She discovers something terrible and tells the Master Chief to find Captain Keyes. Captain Keyes went with several Marines to destroy a take or destroy a supply of Covenant Weapons. The Master Chief discovers the Captain was attacked by a different enemy. They are called the Flood. The Flood are a species of parasite, which can infect bodies and turn them into Combat Forms (which are fast zombies that can use weapons). He finds a flying machine that protects Halo. It is named 343 Guilty Spark. It explains that he should turn on Halo's weapons to destroy the Flood. The Flood want to escape Halo and infect other species. The Master Chief must get an Index to do activate Halo. The Master Chief gets the Index (which can activate Halo's weapons). 