If like me you have not bothered with your Grimoire since the week Destiny launched but perhaps you wanted to learn a little more of the games lore. This guy called Terra Mantis uploaded this really interesting YouTube video that details the history of the Guardians battles against the Hive on Earths moon. Its very well done I highly recommend watching it. He clearly put a lot of effort into it. If Bungie managed to fit this storytelling into the game itself no one would be complaining about the lack of in-game story.