Halo: Combat Devolved (often mislabeled as Halo: Combat UnEvolved) is a shame created by Bungie to show how awesome Master Chief is, and how lame combat will be a few hundred years from now.
The shame takes place in some year in the future where Earth blew up so many times that people live on other planets now. The player plays as Master Chief, a cyborg legend guy who can shoot aliens better than anyone else can. It just happens this shame is about shooting aliens, so he's happy.
It starts when aliens are attacking Master Chief. He blasts them with his shotgun, and blows up the ship (Wait... it was his ship. Oops). He then lands on a weird thing called a Halo. He gets so reckless, he blows everything up. His nanny sends him to his room, but he ignores her, and leaves.
This game comes after Halo: Outta Reach in the Halo Shames timeline.