After the events of the comic tie-in , the Master Chief arrives on Earth in east Africa, where he is found by and the Arbiter. The Chief and company return to a UNSC outpost where Keyes and plan a final effort to stop the Covenant leader, the High Prophet of Truth, from activating a Forerunner artifact the Covenant have excavated. The Chief clears anti-air Covenant defenses so Hood can lead the last of Earth's ships against the Prophet, but Truth activates the buried artifact, creating a portal which he and his followers enter. A Flood-infested ship crash-lands nearby; Elite forces arrive and vitrify the Flood-infected areas of Earth, stopping the threat. Following a message Cortana left aboard the Flood ship, the Chief, Arbiter, Elites, Johnson, Keyes and their troops follow Truth through the portal. Joining them is 343 Guilty Spark, who aids the Chief as he has no function to fulfill after the destruction of his ringworld.
- Halo 3 is a where players primarily experience gameplay from a first-person perspective
- Much of the gameplay takes place on foot, but also includes segments focused on combat
- The balance of equipment and objects in the game was adjusted to better adhere to the "Golden Triangle of Halo": these are equipment, grenades, and melee attacks, which are available to a player in most situations
- Players may some equipment, forgoing the use of grenades and melee attacks in favor of the combined firepower of two equipment