Halo 3
The Halo of the games title is the name of a series of space super weapons built by a mysterious race called the Forerunners a long time before the game. The Halo's are the last defence against another alien race known as the Flood. The Flood want to use every living thing in the galaxy for food. The Halos are actually giant super weapons, that when activated, will kill all life in the galaxy. This is the only way to stop the Flood,to starve them by killing all their food. The Covenant do not know this and are attempting to fire Halos as part of a wrong religious prophecy. In the first Halo game, Halo: Combat Evolved, the humans and Master Chief manage  to stop the Covenant from firing one of the Halos. They also destroy one of them in the process. The destruction of one of the Halos starts an emergency. The Halos can now all be fired from one place, using a device called the Ark. The Covenant, still not understanding the truth, launch an invasion of Earth, where they believe the Ark is buried. This is where the Halo 3 story starts.