This game's plot revolves around a protagonist race called the Forerunners who battle an evil alien race named the Flood. The setting is throughout the Milky Way Galaxy at some point in the distant past. The Flood has infiltrated and spread harm to much of the galaxy, and the Forerunners' weapon of final resort is a series of large structures called Halos. When activated, the Halos destroy most life and therefore cut off the Flood's sources of food. Once the Flood gets starved out, the Forerunners use the Halos to begin restoring the seeds of previously lost lifeforms.