Not really a story line.

Buy it I swear to god it's amazing. My mate got it for me for Christmas we've played 6 hours and haven't even passed the co-op yet. Some of it's really hard but it's good fun playing with mates and the single player is good too.

each campaign has its own storyline

Yeah I wouldn't really know. Played it once (first mission in singleplayer) and havn't gone on it since. More of a game to play with friends, not bots x)

yeah, its hell funny with friends.