Not necessarily. As long as the players would work towards a good story rather than anything else I wouldn't see a problem with knowing the plot. The RP is the journey to the end point which would all be of our own making.

As for the actual storyline, I feel something that starts small, or at least something that the military is tryinn to keep quiet would be our initial premise with the plot unfolding and showing that this terrorist cell or whatever it may be actual could force a large conflict.