H vs. Z

It starts out as a standard zombie-killing game, you and your strike force beating back the hordes of the undead as they slowly take over the civilized world. You take out the undead, you level up in strength and experience, you get better weapons. But in H vs. Z, if you get bitten, or get bitten enough times, you become a zombie. 

And, you stay a zombie. You're now preying on your squad mates, eating their flesh to upgrade your own strength. As a zombie you can "smell" their internal organs so you can pick out the choicest meat needed to upgrade your zombie skills.

The whole time your former mates are begging and screaming your name, saying "FRANK! It's me! Don't you recognize me! FRAAAAANK UGGGGHHhhh ..." 

Your game is stored on a locked file on the hard drive. Reset the game, you're still a zombie. You can't change it. Not until you finish the game. Hey, that's life.