From Halopedia, the Halo wiki
Upon completing the level on Normal, Heroic, or Legendary difficulties, the player will earn the "Cleansing" achievement and 30 Gamerscore; in Halo: The Master Chief Collection, this achievement, now only worth 10 Gamerscore, can also be obtained on Easy. Earning over 15,000 points in the Campaign meta-game will give the player the Exterminator achievement and 10 Gamerscore.
It takes place in Voi, Kenya on the continent of Africa on Earth during the Battle for Earth. Floodgate is the first campaign mission where Sangheili allies other than the Arbiter, Thel 'Vadam, are encountered in Halo 3. It is also one of the shortest levels in the game.
Shortly after John-117 disables the Covenant anti-aircraft battery in the previous level, The Storm, UNSC frigates under the command of Admiral Hood attacked the Covenant-controlled Forerunner Dreadnought. This attempt failed, as the Dreadnought survived unscathed. The Prophet of Truth then activated a Portal to the Ark and his Dreadnought entered the slipspace rift, followed by the rest of the Covenant fleet. Immediately afterward, a Flood-infested battlecruiser emerged from slipspace and crashed into Traxus Factory Complex 09.
In order to eliminate the infestation, John-117 and Arbiter Thel 'Vadam must overload the crashed cruiser's engines, wiping out the entire city and the Flood with it. To do this, they push back through the factory where they just came from, fighting many Flood combat forms and pure forms, the next stage in Flood evolution. They are then aided by the arrival of Rtas 'Vadum and a Sangheili fleet. 'Vadam and John, along with a Sangheili strike team, push forward to the crashed ship, and 'Vadam holds position while John extracts what is believed to be Cortana, stored in a Covenant storage device. This eventually turns out to be only a recording. Inside the crashed Flood ship, John-117 has an unexpected reunion with 343 Guilty Spark. Though distrustful of the monitor's intentions, John accepts Guilty Spark's offer of aid. After the area is evacuated, Sangheili warships begin to glass Voi and the immediate vicinity to prevent further contamination by the Flood.
Onboard Rtas 'Vadum's CAS-class assault carrier, Shadow of Intent, Guilty Spark extracts Cortana's recorded message from the storage device. The recording reveals the existence of a solution to the Flood on the other side of the Portal without firing the remaining Halo rings. The Master Chief convinces Lord Hood to allow an expedition through the Portal to find both Truth and Cortana's solution. A joint human-Sangheili fleet then heads into the Portal.
It Followed Me Home
Hot zone. Weapons free.
Fade in outside the city. The Chief and 'Vadam are standing under the remains of the Covenant AA gun. The sky is choking with thick smog from the crashed cruiser. Faint screeching and growling noises can be heard in the distance.
The Chief and Thel 'Vadam reenter the city.
At this time, a group of Flood combat forms jump from the building on the right.
Flood screeches are heard.
The Chief finds a Marine squad on a loading ramp.
Pod infectors and combat forms (all former humans and Jiralhanae) descend from above and swarm on the Marines at the ramp, killing, then infecting them, forcing the remaining Marines, the Chief, and 'Vadam to kill them.
The Sergeant becomes mutated by a Pod infector.
A Marine proceeds to attack the reanimated Sergeant.
They fight off the wave of Flood.
The Marines accompany the Chief and 'Vadam into a building, where a lone Marine is running away from a horde of Pod infectors.
If the Marine is not saved, the Pod infector dives into his chest, gruesomely transforming him into a Flood combat form. This forces the Master Chief and 'Vadam to kill him immediately.
Marines led by Gunnery Sergeant Stacker are pinned down behind sandbags by a massive horde of Flood. The Chief and 'Vadam engage and destroy the Flood forms in this section of the warehouse.
If the Marines are saved from being killed or infected:
The Chief, 'Vadam, and the surviving Marines head into the next section of the warehouse. Upstairs, two Marines, one wielding a flamethrower, try to combat the Flood as well, but they soon fall to the parasite's overwhelming numbers.
Reaching the other end of the warehouse, the surviving Marines hold position after clearing the area. The Chief and 'Vadam head upstairs onto an upper balcony, optionally picking up the abandoned Flamethrower for use against the Flood.
Proceeding forward, the two find a hole in the floor and drop down it, arriving in the office areas.
The Chief and 'Vadam force their way through another wave of Flood, and find a Marine huddling in a corner of the large room previously used as an Aid Station, surrounded by three dead Marines. The Marine is holding a pistol, and will occasionally aim it at his own head, before lowering it and burying his face in his hands.
With no time to spare, the Chief and 'Vadam continue on ahead to the lake bed.
The Master Chief and Thel 'Vadam enter the lake bed of Voi where the Scarab was previously fought and destroyed.
Shadow of Intent
The Flood has Evolved.
The Chief and 'Vadam make their way to the Sangheili lance.
The Chief, 'Vadam, and the Sangheili fight their way through a massive wave of Flood, including a pair of carrier forms and enter a warehouse. They clear the building of Flood.
The group exits the warehouse. Outside, two Sangheili Phantoms arrive and drop off more Sangheili reinforcements to combat the Flood.
The Master Chief, 'Vadam, and the remaining Sangheili head towards the crash site. Along the way, they encounter Flood pure forms for the first time.
The group fights past dozens of pure forms, combat forms (including former Sangheili), and pod infectors before finally reaching the massive, burning hulk of the crashed Flood battlecruiser. Up an incline, an ominous hole resides in the ship's hull, leading to the interior.
The Chief drops in alone.
Infinite Devil Machine
The Chief drops into a tunnel.
The Chief reaches a large room after passing several dead Sangheili Ultras.
Inside the central chamber of the Flood-infested crashed cruiser, the Chief finds Cortana in a small Covenant hand held memory unit.
Cortana's projection above the holographic unit flickers and then disappears.
Suddenly, the humming of a Phantom's gravity drives are heard, and a Phantom appears over the open chamber. Uncertain of its intentions, the Chief turns and aims his assault rifle at it. 343 Guilty Spark floats down from the Phantom.
A Flood combat form appears to the right behind the Chief, and 343 Guilty Spark casually turns and uses a powerful energy beam to destroy it in a single glancing hit. Spark turns back to the Covenant device and begins to interface it with a beam projected from his central eye.
The Chief grabs 343 Guilty Spark and pulls him away from Cortana's storage unit, disabling the interface beam.
343 Guilty Spark continues trying to project his energy beam into Cortana's storage unit. John again pulls him away, disrupting the beam. He then lets go of Spark and confronts him.
The Master Chief stares wordlessly at 343 Guilty Spark, then holds up Cortana's storage device. 343 Guilty Spark takes Cortana's device away, shooting his beam into it while the Phantom projects a gravity lift. 343 Guilty Spark enters it first and the Master Chief reluctantly follows.
The view cuts to an aerial one over the whole town of Voi at a high altitude. Although clouds obscure the surface, part of the Indulgence of Conviction is still visible. The Phantom flies away from the ship while two battlecruisers of the Fleet of Retribution glass Voi with their energy projectors.
The view then cuts to an aerial one above the Forerunner Portal. Its 'fins' are open and blue energy streams from their tips to the open slipspace portal drifting ominously above the construct. Fighting on Kenya has ceased and the Battle of Kenya has ended with the departure of the Prophet of Truth's fleet through the Portal hours earlier. Four vessels of the Sangheili fleet are gathered above the Portal and 343 Guilty Spark's Phantom arches to the largest of them: the assault carrier Shadow of Intent.
Inside the bridge of the Shadow of Intent, Sangheili and humans are gathered around a long holographic table, with Shipmaster Rtas 'Vadum floating on a gravity throne at one end with his Sangheili attendants, Master Chief and Arbiter Thel 'Vadam in the middle, and Fleet Admiral Hood and Commander Miranda Keyes at the far side of the table. All are silent except 343 Guilty Spark, who is bobbing at the center of the table and interfacing with Cortana's memory unit with his projector beam.
343 Guilty Spark glances at 'Vadum and Lord Hood
As Spark finishes his work, the storage device lights up, and a holographic projection of Cortana springs up.
Cortana's image flickers and stalls. The Chief turns to Rtas 'Vadum.
343 Guilty Spark jolts the device with another beam.
Suddenly, Cortana's skin lights up. She clutches her head, gasps in sudden pain and collapses onto the "floor".
Cortana's recording ends and she is left staring with desperation at the Chief.
Sangheili around the room raise their forearms and roar in agreement.
Lord Hood angrily raises his fist at 'Vadum, but Miranda Keyes interrupts.
While Hood and Keyes argue, the Master Chief bends down and observes Cortana's pained, frozen figure, still collapsed on the holographic table.
Lord Hood turns, looks wordlessly at Miranda Keyes, and then exits the bridge.
Inside the Shadow of Intent's hangar bay, both the Sangheili and the UNSC are getting ready for battle. While a pair of Sangheili Phantoms drift into the hangar on the far side, two Sangheili Banshees glide past in the opposite direction. On the ground, two Warthogs race to the UNSC Forward Unto Dawn's loading bay, drawing near a group of Marines clustered around some UNSC crates. At uneasy rest opposing the Marines are a Sangheili Major sitting on a Ghost, along with a few Special Operations Sangheili. The Major's particle beam rifle is resting next to the Ghost. Sergeant Johnson, with his arm full of "stolen" Covenant carbines brushes past the wordless Sangheili, stopping only to scoop up the Major's rifle. The Sangheili, resentful at this non-permitted collection, rise challengingly around Johnson. Johnson ignores them and continues walking.
Nearby, a pair of ODSTs are joking with each other when the Arbiter passes by with an acquired rocket launcher over his shoulder and a flamethrower in his left hand. The ODSTs and a pair of nearby Marines watch the Arbiter in awe, when 343 Guilty Spark drifts through the Marines in tow behind 'Vadam, pausing momentarily to glance at one Marine, leaving him speechless. Johnson rejoins the Marines, and begins to hand out carbines.
The scene cuts to the fore of the hangar, where a Pelican dropship lays before the hangar's energy field. The Forward Unto Dawn is stationed nearby. The Chief and Miranda Keyes stand at attention before Lord Hood, who looks at them grimly from the rear of the Pelican as the troop bay door shuts and the Pelican takes off. Keyes glances at the Chief wordlessly, and the two leave, going off to their own business.
Hood's Pelican arcs away from the gathered Sangheili fleet as the Shadow of Intent powers its engines and thrusts into the Portal, followed by the rest of the fleet.
Fades to black.
During the development of Halo 3, Floodgate was originally intended to be a much longer mission, with the section inside the crashed cruiser given considerably more play time. Ultimately, this was deemed too long and badly-paced, and the level was cut-down by around 75%. At the same time, the early layout for the level Cortana was scrapped and that mission increased in scope by around 75% - due to issues with development, the geometry cut from Floodgate was transferred to Cortana.