I rely upon Sun Tzu and Dwight D. Eisenhower: in war (or games), plans are useless but planning is essential.
In my experience, no scenario outline has ever survived contact with the PCs. This is a feature of RPGs, not a bug; once you realize the Zen of this, you can plan with the confidence that all your hard work will pay off the first time the players blindside you and totally wreck your lovingly-crafted plot... and you just roll with it, pulling some plausible-yet-improvised-on-the-spur-of-the-moment new plot point out of hard vacuum totally nonchalantly, subsequently convincing them that this angle was your plan all along.