As great as the games themselves were/are, Halo's story is a convoluted one. Really. It's a mess. Never before has this been better exemplified by these two documentaries (except maybe for that part halfway through Halo 3 where you wonder what the fuck is going on), put together by Bungie, to serve as a primer for the series' third game. Yes, two of them. Together, they run for over 16 minutes.
Shouldn't take that much explaining! "Aliens are bad, you have to kill aliens to save all the people" should really take about 3 seconds. Any longer than that and - like Bungie have - you're going to run into trouble.