There are games that were praised highly and already get awards long before their release. Halo surely is such a game. That's not too far-fetched, Halo has a lot of potential, and not only in its graphics. You're pretty much on your own as to what you do in this game, but there is of course a plot behind the whole thing. Halo helps to kill off the typical, boring plots we always get, it's quite a huge plot, but at first it only scratches the surface.
Halo is set far in the future where humans have started to leave Earth to colonise other planets.
After some time though they bump into the Covenant, a agressive group of alien races. These aliens slowly destroy human colonies on their way towards Earth. One of the last human ships tries to distract the Covenant from Earth by making them go a different direction. The Covenant catch up with the humans and destroy their craft, but marines and equipment are saved. The Covenant land on Halo too, and guerilla warfare between the humans and aliens starts.