In the 26th century, the Covenant, a powerful alliance of alien races, launches a surprise attack on Earth. Master Chief defends the planet's orbital stations before pursuing the Covenant fleet to a second Halo installation. Meanwhile, a disgraced Covenant Elite commander is branded a heretic for failing to prevent the first Halo's destruction and given the suicidal rank of Arbiter. As both warriors pursue their separate missions, they uncover a brewing civil war within the Covenant and a parasitic threat that endangers all sides.