There are three main Halo campaign games so far: Halo Combat Evolution, Halo 2 and Halo 3. In all three games Master Chief has to fight the Covenant and the Flood but in different ways. In Halo Combat Evolution during the whole game you are on Halo, which you found while trying to escape from a Covenant fleet. Your main goal is to destroy the weapon before it can be fired. Then in Halo 2, the Covenant have found Earth but don’t know about the humans on it and bring a small fleet that is easily driven back but is followed by the UNSC to another Halo ring. At the end you have to kill the brute leader, a newly added Covenant troop, but lose Cortana. Halo 3 you are back on Earth and the Covenant have sneaked on and found an artifact that will help them fire all the Halo rings. During their escape, the Flood crashes on Earth. After you kill the Flood you decide to follow the Covenant to the Ark; a control room that can control all the Halo rings. You have to destroy the ark and save Cortana from the Flood leader. There are more games added too, like Halo 2 Multiplayer Map pack, which adds playable maps to the multiplayer mode on Halo 2. Also Halo Wars allows you to take control over a huge army of ether the Covenant or the UNSC. You don’t play an actual person but control them like in Command and Conquer. Also a new campaign, called Halo 3 ODST, came out on September 22 where you play as one of the UNCS soldiers and not Master Chief.