Halo 2 takes place in the 26th century. Humans, under the auspices of the United Nations Space Command or UNSC, have developed faster-than-light slipspace travel and colonized numerous worlds. According to the game's backstory, the outer colony world of Harvest was decimated by a collective of alien races known as the Covenant in 2525. Declaring humanity an affront to their gods, the Forerunners, the Covenant begin to systemically obliterate the humans with their superior numbers and technology. After the human bastion at the planet Reach is destroyed, a single ship, The Pillar of Autumn, follows protocol and initiates a random slipspace jump to lead the Covenant away from Earth. The crew discovers a Forerunner ringworld called Halo, which the Covenant wants to activate because of their religious belief that the activation of the ring will bring about a "Great Journey," sweeping loyal Covenant to salvation. Leading a guerilla insurgency on the ring's surface, the humans discover that the rings are actually weapons of last resort built to contain a terrifying parasite called the Flood. The human supersoldier Master Chief Petty Officer John-117 and his AI companion Cortana learn from Halo's AI monitor, 343 Guilty Spark, that activation of the Halos will prevent the spread of the Flood by destroying all sentient life the parasite can subsist on in the galaxy. Instead of activating the ring, however, the Master Chief evades Guilty Spark and his robots and detonates the Pillar of Autumn's engines, destroying the installation and preventing the escape of the Flood. The Master Chief and Cortana race back to Earth to warn of an impending invasion by Covenant forces.