Just after the Covenant fleet of spacecraft leave the portal, a big ship crashes into the city of Mombasa. This ship has been taken over by the Flood. The player as Master Chief, must fight their way into the ruined spacecraft and recover the robot Cortana who can tell them the way to re-open the portal and follow the Covenant. Once the player is successful, the humans re-open the portal and head after the Covenant to stop them firing the Halos. Joining them is 343 Guilty Spark, one of the robots who helped build the destroyed Halo from the first game. Once through the portal the player and his allies discover the huge, planet-sized structure known as the Ark, where all the Halos can be fired from. Led by 343 Guilty Spark the player makes their way to the centre of the Ark where the Covenant leader, the Prophet of Truth is preparing to fire.