Seth Godin on Plans and Missions

I really do like Seth Godin, but I'm not quite sure what he was saying with this post:

Plans are great. But missions are better. Missions survive when plans fail, and plans almost always fail.

One of my colleagues pointed out that:

Missions are toothless without plans. Plans move missions from rhetoric and paper to reality. Plans make someone accountable to deliver on the mission by a specific date.

Seth's post suggests that Plans and Missions are substitutable.  But I don't think that is what he means. 

While it would be nice if Seth could expand on what he meant, I think he is just saying that both are important together.  Missions can't get the job done but plans do.  Yet plans alone won't keep you focused and motivated but Missions will.