Halo 5: Guardians launches next week and if you're bit rusty on your Halo canon you might want to watch this video to get up to speed on the history of Spartan 117. I tell you this because the game assumes that you know what has already gone on and does not take any time to get you ramped up on the story.
If you're planning on picking up the game spend 11 minutes of your time to get caught up on the franchise as the video covers every Halo property including the comic books, movies, as well as the games. There's a lot of great stuff in here and it will definitely make your experience a bit better.