Variation

the item whose presence always pushes the Overall Story forward

The Overall Story Catalyst is what creates breakthroughs and seems to accelerate the development of the Overall Story. In both the Overall and Main vs. Impact Stories there occur dramatic "log-jams" when things seem to be approaching a halt. This is when the Catalyst is necessary, for its introduction will either solve the puzzle that's holding things up or else make the puzzle seem suddenly unimportant so the story can continue.