When I set out to write the Embers of War and Ack-Ack Macaque trilogies, I had to decide how to structure the plot across three books. So, I took a lesson from Star Wars.
Book One establishes our heroes and sets up their world and the background to the coming conflicts, but it’s also an adventure in its own right, with our heroes triumphing at the end.
But then along comes Book Two, and everything gets turned upside down. Gains made in the first book are lost; the characters find the rug pulled out from under them; Luke finds out who his real father is…
So, the start of Book Three finds our characters at their lowest ebb. Their plans are wrecked and they’ve seemingly lost everything. And that’s right when the forces opposing them seem to be ascendant. A new Death Star is built. Han is delivered to Jabba. Leia becomes a slave. They have reached rock bottom and the only thing to do now is fight back. Their trials in Book Two have changed them, and they have new strengths and new goals. They understand themselves better.
The end of Book Three pulls together all the threads from One and Two, and resolves all the character arcs to create a satisfying conclusion.
To reduce it to a simple diagram: