At the end of Halo 3, Master Chief saved humanity by activating the Halo superweapon to wipe out the parasite army know as the Flood. Following that he entered a cryonic sleep and told artificial intelligence companion Cortana to "Wake me, when you need me".
Five years later everyone's favourite Spartan is woken up, and this time he has to protect mankind from imminent extinction yet again, and save Cortana from losing her mind. But first, he must fight his way out a strange new planet he crash landed on after defeating the Flood. 4/5