The Battle of Installation 04 was an engagement between the UNSC Pillar of Autumn and the Covenant Fleet of Particular Justice on and around the Forerunner megastructure known as Installation 04, or just Halo, in the Soell system. Master Chief Perry Officer John-117 along with AI Cortana made landfall in order to escape the Covenant attack on the ship as well as to investigate the newly discovered structure. While the Pillar of Autumn ended up crashing on the interior surface of Installation 04, many of the ship crew were able to escape and reconnoitered on the surface.
In the process of investigating the Halo, Master Chief managed to gain access to the control room, and a squad of marines were able to open what they thought was a weapons cache. The weapon's cache turned out to be the area of the Halo that kept the Flood quarantined from everywhere else, and the actions of the marines allowed the Flood to escape.
Sensing a need to stop the flood and the presence of a reclaimer, the facility's monitor, 343 Guilty Spark, made contact with Master Chief to try and activate the Halo array in order to eliminate the Flood. Master Chief was nearly successful, but was stopped by Cortana who informed John the purpose of Halo was to eliminate both the flood forms and their food, any and all sentient life.
Cortana and Master Chief devised a new plan to destroy the Halo via the self-destruction of the Pillar of Autumn. The plan included retrieving Captain Keyes to get security access to the self-destruct function, but in the process the Captain was discovered to have been consumed by the Flood. John was able to collect the captain's neural interface from what remained of Keyes, and headed to the Pillar of Autumn.
Master Chief and Cortana were ultimately successful in destroying the Pillar of Autumn and Installation 04, despite the attempts by 343 Guilty Spark to stop their plan. While a handful of UNSC personnel including John and Cortana were able to make it off the installation before its destruction, most UNSC forces and any Covenant forces on the ring, were killed.