How Fourth Wall Studios hid an artistic radio drama inside Halo 3:ODST

In this case, Sadie’s Story exists entirely within the ODST game. It’s as if an alternate reality game were embedded in the fictional reality of the console game.

http://games.venturebeat.com/2009/10/09/how-the-startup-fourth-wall-studios-created-a-game-inside-a-game-for-bungie/