Having survived the destruction of the Dawn, Master Chief and Cortana search for a way off of the mysterious Forerunner planet.
Requiem is the 2nd level in the Halo 4 campaign. It was first announced along with the rest of the achievements for Halo 4. This is the first level that takes place on Requiem. The level focuses around Master Chief’s efforts to find a way off the planet and return to Earth. (Source)