This article is about the Halo 3 campaign level. For the human AI, see Cortana.
Upon completing it on the Normal, Heroic, or Legendary difficulties, the player will earn "The Key" achievement and 40 Gamerscore. Earning over 15,000 points in the campaign meta-game on this level will give the player the Orpheus achievement, and 10 Gamerscore.
With the Prophet of Truth dead and the Covenant defeated at last, the Master Chief must now infiltrate the Flood-infested High Charity and rescue Cortana, who possesses the Activation Index needed to activate the new Halo. The Chief flies into High Charity in a Banshee and journeys into the depths of the city, searching for his lost AI companion. During this mission, not only does the Chief encounter stiff resistance from the Flood, he is constantly plagued by disturbing visions of Cortana being tortured, as well as taunts and threats from the Gravemind. Despite this, he finally locates and retrieves Cortana, finally fulfilling the promise he made to her before departing for Earth. The Chief then overloads High Charity's backup reactor to destroy the city. The Arbiter unexpectedly shows up and aids the Chief in his escape. The pair find a crashed Pelican and use it to escape High Charity just before the city is destroyed.
The Chief drops down onto a wide platform from a hole above ('Vadam follows him in co-op)
(Co-op Change) Johnson (COM): "The Shipmaster will do the same with the Elites."
The Chief shakes off some organic material sticking to his boot.
The Chief begins his journey through the hive, killing any Flood in his path.
The Chief drops down a porta.
The Chief passes through the area where the High Council Chamber once was. Exploring further, the Chief goes to an outside area where he finds a crashed, but intact Pelican, possibly from the UNSC In Amber Clad.
As the Chief goes deeper into the hive.
Cortana begins to show signs of rampancy.
The Chief fights through more narrow, Flood-contaminated tunnels.
As the Chief nears a large room (with the route to the reactor room at the top):
The Chief continues to the reactor room.
The image of Cortana suddenly turns a tinge of green. She clutches her head and writhes in pain.
The Chief heads through a tunnel littered with bones and skulls, and enters the reactor room. He sees one of the four reactor pylons malfunction and overload. The Chief fights his way through the reactor room, and exits through a door on the other side. He enters yet another hallway.
The Chief begins to reach the end of the hallway.
The Chief rushes to the end of the hallway and enters the former Sanctum of the Hierarchs. There he finds Cortana lying prone in a stasis field in a center podium. He bashes the shield until it finally fails. Cut to black.
The Master Chief leans closer towards the panel Cortana's on. The AI lies pitifully on the podium, pain and shame on her face. The animated code that usually flows across her skin is absent.
The Chief kneels right next to Cortana.
Cortana looks up at the Chief. She starts to glow.
Cortana slowly gets up. As she does, she begins to glow more brightly and the lines of code and equations begin to fall over her surface again. She opens her palm and a hologram of the Index appears above it.
The Chief gets back on his feet.
The Chief takes Cortana's old data chip out of his helmet and holds it in front of her. Cortana touches its core and uploads herself into the chip. The Chief inserts Cortana back, at long last, into his armor.
Nor Hell A Fury…
Shoot your way out.
The Chief heads back out the hallway.
The Master Chief returns to the reactor room. Flood forces, now aware that Cortana is in possession of the Index, start attacking. The Chief stands his ground and defeats them.
The Chief heads to the center of the reactor room and accesses the main controls, exposing the three remaining reactor pylons. A new wave of Flood swarms the reactor room, attempting to stop the Chief. However, he successfully destroys all three reactor pylons.
Alarms blare. Explosions erupt throughout the reactor room.
The Chief leaves the reactor room and races through High Charity, which is now rapidly falling apart.
If the Chief takes too long:
When the Chief reaches the halfway point to escaping High Charity:
The Chief reaches a blocked hallway.
If the Chief still can't find his way out:
The Chief finds his way back when…
The Chief and Cortana find the Arbiter battling Flood with a flamethrower (occasionally, Thel will use his energy sword instead).
The Chief and 'Vadam clear the immediate area of remaining Flood, then head outside.
The Chief and the Arbiter board the Pelican the Chief discovered earlier. The Chief uploads Cortana into the Pelican's holotank and starts up the engines. One of the Gravemind's tentacles reaches for the dropship, but the heat from the thrusters forces it to retreat. The Pelican takes off, smoke pouring lightly from the left engine. The tentacle comes back and tries to ensnare the dropship, but the Pelican pulls free of its grip. The Pelican barely escapes as High Charity's reactor goes critical, consuming the planetoid in a massive explosion. The Pelican makes its way to the new Halo ring.
The original idea for the level Cortana was entirely scratched. Originally, the level had a proper High Charity geometry completely different from the geometry seen inside the Flood-infested cruiser in Floodgate. The player had to follow Cortana's footprints to know where to go. As to why Cortana would be able to leave such footprints, Bungie designer Dan Miller likened the situation to her mysterious appearance in the closing cutscene of The Covenant. Once Cortana was rescued, the player was to encounter a Scarab with damaged hind legs. The Chief would have to insert Cortana so she could take control of it. Then, the Gravemind would appear and a battle between him and Cortana's Scarab would ensue. The Chief would have to hold his position on the Scarab by fending off invading Flood forms and shooting off the Gravemind's tentacles that would try to latch on the Scarab. Eventually, the entire level —including its specific High Charity geometry— had to be cut from the game's development.
As a replacement for the deleted Cortana level, Floodgate was split into two distinct levels, the latter part of the mission becoming the new Cortana level. Thus, the geometry in the released Cortana level is the same as the one of Floodgate, and there is no unique High Charity geometry in the final game. The second part of Floodgate —now the new and final Cortana level— was acknowledged by Bungie as being a bad mission, as it did not have its own level designer.