Halo 3: ODST , the fourth installment of the halo series, is its own episode that branched off the storyline of the first three. In ODST you are a special soldier sent to investigate battlefields after the war is over. As you investigate the cities involved in the war, you come across different pieces of evidence which launch you into a separate level re-living the event which led to the piece of evidence you found to have come to that place. So half the game you are the soldier investigating, and the other half you are re-living experiences. While many people were not especially fond of ODST I liked it very much and think it is definitely worth playing.