Outbreak at the Containment Facility

The Battle of the Containment Facility was the turning point of the UNSC and Covenant's involvement in the Battle of Installation 04, where the two sides proved unprepared for a third enemy: the Flood.

Events
Some time before Keyes' arrival, the Covenant had gained control of the containment facility and accidentally released the Flood. While the upper levels were cleared, the lower levels were infested. The Sangheili commanders were lost in the process.

After being informed by a captured Sangheili that the facility held weapons, Captain Jacob Keyes, along with 2nd Squad made their way to the structure, expecting to find a weapons caché that could be of some use against the Covenant. They landed on the D77-TC Pelican dropship Victor 933 and proceeded to infiltrate the structure. They later met up with Staff Sergeant Avery Johnson's "Fireteam Charlie", who followed behind on their own pelican.

While exploring the facility, Keyes' team eventually came across a sealed door, which was locked down by the Covenant to prevent the escape of the Flood. With increased curiosity, Keyes ordered that the door be opened. Finding nothing sinister about the room initially, some of the men become nervous when they head a strange hissing sound, followed by Second Squad, who were exploring a lower level, reporting that something "not Covenant" was "tearing" through them. They would soon encounter what that something was, as a number of Flood infection forms broke through their containment cells and began to attack the team, attempting to convert them. Of the team, only three escaped conversion - Sergeant Johnson, who managed to leave the room in time; an unnamed marine who "played dead", and Mendoza, whose corpse was abandoned by the parasite.

Meanwhile, a crashed Spirit dropship in the swamps was comandeered by the Flood, who used it to escape the ringworld and attack the Fleet of Particular Justice's ships.

Later on, Master Chief Petty Officer John-117 arrived near Victor 933's crash site on board Echo 419, following Fireteam Charlie's route while being watched by the Flood, whose Marine IFF transponders flickered on and off the SPARTAN's radar. After entering the Containment Room, he found Private Wallace A. Jenkins' helmet, which contained a built-in video camera. After seeing the gruesome events through the eyes of Jenkins, the remaining Flood spewed out of nearby doors. John-117 proceeded with the remainder of Second Squad, combating Infection and Combat forms in the open, after being advised by Foe Hammer to take refuge at a tower out in the swamps.

The survivors went through the swamp in search of a tower, which was their designated evac site for Echo 419. After making it to the structure, the Marines were aided by a mysterious ally - floating constructs known as Sentinels. The Sentinels, commanded by the Monitor 343 Guilty Spark, defended the humans, eliminating any Flood in the vicinity. The Monitor then teleported the Master Chief to it, referring to him as "Reclaimer", and then taking him away with him to the Library.