Halo 3 takes place just after the events of Halo 2. High Charity, the Flood infested Covenant ship/city, is falling apart. In part of it Master Chief jumped out of it and landed somewhere in Africa. Sergeant Johnson and a group of marines find you in the jungle. The Covenant have landed on Earth. You and the Arbiter have to fight through the jungle to rid the nearby town Voi of the Flood. A group aliens led by the Shipmaster, who are not part of the Covenant, glass (destroy) Voi to kill the Flood. You go to a hidden place where the Prophet of Truth is trying to fire all of the Halo Rings at once. Halo 3 is my personal favorite Halo game. It is also argued as the best Halo game. It came out in 2007 on the Xbox 360 and PC.
All About Halo 