In the end of Halo 3, Master Chief swirls in space with Cortana. He couldn't actually die because he's a super soldier and doesn't need to eat and can breath in outerspace, but he's back! But when he lands on earth he finds every human exterminated, even the Elite! The great brain and the robots he invented are behind it. Now you must get back to your spaceship, go to the place the robots are being made and destroy it. Good luck, Master Chief!
