Six months after being defeated in battle by Atriox, the leader of the Banished mercenary organization, the Master Chief is recovered from space by a stranded UNSC pilot. Together they travel to Zeta Halo, a damaged Forerunner ringworld now under Banished control. On the ring, Chief discovers the Weapon, an AI created to capture and delete his former partner Cortana. Though the Weapon reports success in her mission, she has inexplicably failed to self-delete as programmed. As Master Chief rallies scattered human forces and contends with Escharum, the Banished war chief leading operations on the ring, a new threat emerges: the Harbinger, an alien who claims her imprisoned people were unjustly sealed away by the ancient Forerunners. She seeks to rebuild a facility that will free them, and the Banished intend to weaponize the Halo ring itself against humanity.