S01E01Origins Part 1

Beginning aboard the Forward Unto Dawn, on which Cortana and Master Chief were stranded after the events of Halo 3, Cortana begins by musing of her seven years of existence and that she has learned a great deal more about the Forerunners than was previously known. The story then shifts to a Forerunner planet before the invasion of the Flood, and then leads up to infection of the planet and it's eventual orbital bombardment.