Setting: Installation 04 (a Halo ring), shortly after Reach’s fall.

Lore Summary:
Fleeing Reach, the Pillar of Autumn, carrying Master Chief (John-117) and Cortana, crash-lands on a ringworld called Halo, built by the ancient Forerunners. The Covenant pursues, believing the ring holds religious significance. Chief and Cortana discover Halo’s purpose: it’s a superweapon designed to wipe out the Flood, a parasitic lifeform that assimilates sentient beings. After the Covenant accidentally unleashes the Flood, Chief battles both enemies. To prevent galaxy-wide destruction, he and Cortana detonate the Autumn’s reactors, destroying the ring, and escape in a Longsword fighter. Cortana remarks, “It’s not over.”

Key Themes: Survival, discovery of ancient threats, and human-AI partnership.
Key Characters: Master Chief, Cortana, Captain Keyes, 343 Guilty Spark (Halo’s monitor).

Setting: Earth, High Charity (Covenant capital), and Installation 05 (another Halo ring).

Lore Summary:
The story alternates between Master Chief and the Arbiter (Thel ‘Vadam), a disgraced Covenant Elite. After the Covenant attacks Earth, Chief repels the assault but pursues a Covenant ship to Installation 05. The Arbiter, punished for failing to protect Installation 04, is sent on a redemption mission by the Covenant’s Prophet leaders. Both uncover the Prophets’ deception: the “Great Journey” (activating the Halo rings) will annihilate life, not ascend it. The Arbiter allies with humans after the Prophet of Truth betrays the Elites, favoring Brutes. The Flood’s Gravemind manipulates events, and Cortana stays on High Charity to stop Truth. Chief returns to Earth, vowing to “finish the fight.”

Key Themes: Betrayal, alliance, and questioning blind faith.
Key Characters: Master Chief, Cortana, Arbiter, Prophet of Truth, Gravemind, Tartarus.

Setting: Earth, the Ark (a Forerunner installation), and a replacement Halo ring.

Lore Summary:
Master Chief and the Arbiter unite to stop the Prophet of Truth from activating the Halo array via the Ark, a Forerunner structure outside the Milky Way. After defending Earth, they follow Truth through a portal. Cortana, rescued from the Flood-infested High Charity, provides critical data. The Flood, led by the Gravemind, briefly allies with them to stop Truth but betrays them. Chief and the Arbiter kill Truth, destroy the Ark, and activate a new Halo ring to eliminate the Flood. Escaping, their ship splits; Chief and Cortana are stranded in space, entering cryo-sleep as Cortana says, “Wake me when you need me.”

Key Themes: Redemption, sacrifice, and the end of the Covenant war.
Key Characters: Master Chief, Cortana, Arbiter, Prophet of Truth, Gravemind, 343 Guilty Spark.

Setting: New Mombasa, Earth, during the events of Halo 2.

Lore Summary:
You play as “the Rookie,” an Orbital Drop Shock Trooper (ODST) in a squad dropped into Covenant-occupied New Mombasa. The story unfolds non-linearly as the Rookie, separated after a botched drop, explores the city and relives his squad’s missions via flashbacks. The team’s objective involves securing a critical AI, the Superintendent, while fighting Covenant forces. The narrative highlights human grit and teamwork amidst Earth’s invasion. The squad reunites, escapes, and learns of the Prophet of Truth’s plan to activate a Forerunner portal to the Ark.

Key Themes: Human resilience, urban warfare, and camaraderie.
Key Characters: The Rookie, Buck, Dare, Vergil (Superintendent AI).

Setting: Reach, humanity’s primary military stronghold, 2552.

Lore Summary:
You play as Noble Six, a new member of Noble Team, a squad of Spartan supersoldiers serving the United Nations Space Command (UNSC). The Covenant, a fanatical alien alliance dedicated to humanity’s extinction, invades Reach. Noble Team undertakes desperate missions to repel the attack, including defending cities, escorting key personnel, and delivering the AI Cortana to the ship Pillar of Autumn. As the Covenant overwhelms Reach, most of Noble Team sacrifices themselves. Six makes a final stand to ensure the Autumn escapes, but Reach falls, marking a catastrophic loss for humanity.

Key Themes: Sacrifice, duty, and the cost of war.
Key Characters: Noble Six, Carter, Kat, Jun, Emile, Jorge, Cortana, Captain Keyes.

Setting: Requiem, a Forerunner shield world, four years after Halo 3.

Lore Summary:
Master Chief and Cortana awaken when their drifting ship enters Requiem. They encounter the Didact, a Forerunner leader revived from stasis, who seeks to dominate humanity using the Composer, a device that digitizes organic life into AI constructs. Cortana, suffering from rampancy (AI degradation), struggles to support Chief. A Covenant splinter faction led by Jul ‘Mdama worships the Didact. Chief allies with the UNSC Infinity to stop the Didact’s plan to digitize humanity. Cortana sacrifices herself to save Chief, defeating the Didact. Chief, grieving, vows to fight on.

Key Themes: Loss, AI sentience, and new threats post-Covenant.
Key Characters: Master Chief, Cortana, Didact, Captain Del Rio, Lasky, Jul ‘Mdama.