"Mann vs. Machine is a new co-operative game for Team Fortress 2 that lets you and five friends wage a desperate battle to stop a lethal horde of robots from deploying a bomb in one of Mann Co.’s many strongholds. Take advantage of breaks between waves to upgrade your abilities and weapons. Survive all the waves in any of a variety of missions to earn incredible loot." -Mann Vs. Machine Update Page: http://www.teamfortress.com/mvm/

Now, after watching the video you might be wondering why -INSERT UNUSUAL EVENT HERE-. You might want to have a gander at these 2 comics:

Part 1: http://www.teamfortress.com/bloodbrothers/
Part 2: http://www.teamfortress.com/fateworsethanchess/

Map preview: http://www.teamfortress.com/mvm/mannworks/

Bots: http://www.teamfortress.com/mvm/machines/