"Stop the Infestation. Find Cortana."
Floodgate is the sixth campaign mission of Halo 3 and the first Halo 3 mission in which the Flood appears.
It takes place in Voi, Kenya on the continent of Africa on Earth during the Battle of Earth. Floodgate is the first campaign mission where Sangheili allies other than Thel 'Vadam are encountered in Halo 3. It is also one of the shortest levels in the game.
Shortly after John-117 disabled the Covenant Anti-Aircraft Battery in the previous level, The Storm, UNSC Frigates under the command of Lord Terrence Hood attacked the Covenant controlled Forerunner Dreadnought. This attempt failed, as the Dreadnought survived unscathed.
The Covenant Loyalists then activated a Portal to the Ark, temporarily disabling the Frigates. The remnants of the Covenant Loyalist fleet then followed the Dreadnought into the Portal.
With the UNSC fleet disabled, and the Anti-Aircraft Battery out of commission; a Flood-infested Covenant battlecruiser suddenly jumped out of Slipspace nearby and crashed into Traxus Factory Complex 09.
Immediately after the crash of the flood loaded battle cruiser, the infection spreads around the city, infecting every dead body in it's grasp.