The human race and the UNSC are at war with the alien race, the Covenant, who are bent on wiping out humankind. After the huge battle at Reach, one of the last remaining human strongholds, the SPARTAN soldiers were wiped out. Captain Keyes, a general of Earth, attempted to crash land the Pillar Of Autumn on Halo with the master chief. He jumps ship with the rest of remaining crew, just at the last minute in a rescue pod with a stowaway on board. An Elite, a Covenant soldier whose orders are to attempt to kidnap the Captain, hides on board camouflaged by a cloaking device.
The remaining survivors set up a makeshift base and Master Chief, and the others discover that the Covenant view Halo as sacred a sacred place. They think this because to them Halo serves an unknown purpose. The Covenant wants to steal our technology and they suspect that Halo is not just a huge space station. Equipped with artificial gravity, landscape and an atmosphere similar to that of Earth’s, but we have a massive weapon storage contrary to Earth. Master Chief and the rest of the human military on Halo discover that Halo has a number of dark secrets.
The most significant of which is that Halo is a weapon but not quite as the Covenant or the humans first think. There is also another alien presence on Halo, one which the human and the Covenant would rather not encounter. They are here as we speak, trying to uncover our secrets.