Let me start by saying that I don't plot my campaigns game-by-game. I write out the major events of the over-arching story, then write each week's game in the week leading up to game night, deciding then how much of the overall plot will be revealed. So this won't be another boring breakdown of pacing or combat vs. dialog or any of that overthinking nonsense. This is simply a method to flesh out the over-all storyline of the campaign around which you'll set your individual games.
What is the Double-Tap
Simply, the double-tap is hitting the players twice, in quick succession, with two huge reveals. Rarely are these reveals back-to-back in the same play session, but they do come back-to-back in terms of the overall story.
In the double-tap structure, the players are given a problem to solve, a singular goal around which the campaign hinges. As they near the end of that goal you hit them with the first shot - the problem is much worse, much bigger, and has a much larger lead on them than they actually thought. In traditional three act formula, this is known as the climax or second turning point, in Jo-ha-kyū it's the break, where things suddenly accelerate out of control; the heroes, who until now have trudged through ever complication thrown at them, suddenly find themselves against overwhelming odds against which they cannot prevail and the clock is ticking down even faster. This is standard stuff in adventure stories.
In the double-tap, the characters barely overcome their challenge, are victorious over the villain, and pick up their things to walk off into falling action and denouement - only to find that a bigger, meaner, more evil antagonist has been there the whole time and bam they're hit with another climax. This is terrible in a book, even worse in a movie, but played right in a gaming campaign and you can create a massive oh shit moment.
What follows are the five steps toward running a double-tap campaign.