Bent on Humankind's extermination, a powerful fellowship of alien races known as the Convenant is wiping out Earth's fledgling interstellar empire. You and the other surviving defenders of a devastated colony-world make a desperate attempt to lure the alient fleet awat from Earth. Shot down and marooned on the ancient ring-world Halo, you begin a guerilla-war against the Covenant. Fight for humanity against an alien onslaught as you race to uncover the mysteries of Halo.