If you want the story, read the book first, otherwise I wouldn't bother with this game
This game tries to emulate the new FPS must with the 2 weapons only, unfortunately 1 has to be a Tau (the race of the main character) weapon so you are pretty restricted in the middle of the game if you wanted to carry a plasma gun and a bolter and in the end the Tau weapon is the only one you'll need to use. A melee attack is added in the form of a knife (which you have to switch to like a gun), but I'd be surprised if you ever used it.
The story, although very intriguing in the book, is virtually non-existent in the game. I started rereading the book because I couldn't tell what was going on in the game. Several objectives did not make sense to me until I got to that part in the book and then I realized why I was supposed to do what I did.
All in all this game is a decent shooter if you read the book, but otherwise it's just a bad game.