The Halo 3: ODST campaign is a first-person shooter about the struggle for the survival of mankind. It is the story of a war between two species, in which the most powerful ones on the planet, the Galactic Union, is forced to send its army to fight against the evil forces of the enemy, the Organization of the United States of America, aka The Covenant.

As the Master Chief, you are the leader of the military force that fights against them. You have to protect the planet from their attacks. In the course of the battle, you will meet many other characters and learn the history of the war, the planet, and the universe.