Valve are dressing it up, but Team Fortress 2‘s new Mann vs. Machine mode is really just an adaptation of the co-op Horde mode that’s become popular in FPS games in recent times. TF2′s implementation involves the addition of a new grey team of robots (hence the “Machine” from “Mann vs. Machine”) with varying abilities analogous to different player loadouts.
The new mode was pre-announced thanks to an extensive alternate reality game that started around the time of the Meet The Pyro video, and was propelled when two hidden links appeared on the TeamFortress.com website. Those links led to previously unrevealed lore that, along side the two brothers who run the RED and BLU organisations that form the basis of TF2′s two teams, a third brother, Grey, was also born.
The whole thing is detailed in completely hilarious comic form over at the TF2 website, and is well worth checking out, even if you don’t still play TF2. If nothing else, the video is pretty great: