Plot, of course, combines a few very specific elements together: the exposition (when we learn about the characters and setting), the inciting incident (sometimes called the conflict, when we learn about the one action that begins the suspense), the rising action (where the suspense builds), the climax (the highest point of the tension), the falling action (as the characters world starts to return to normal), and the resolution/denouement (when the conflict is resolved and the conclusion is reached). The Nation, by Pratchett, has a very definite plot in regards to the specific elements above.
The exposition of this particular novel is when we learn about the two main characters of Ermintrude and Mau in their home of their home island.
There is a most definite inciting incident (or conflict) here where Ermintrude and Mau are washed away by a tidal wave and survive (unlike the other members of their home island and very few from neighboring islands).
The rising action involves much of the plot here. Ermintrude and Mau begin cooperating for survival reasons with all of the other survivors from other islands. These cooperative activities make the full rising action.
The climax happens when the survivors and new members of the Nation unearth, excavate, and explore a spiritual cave where they discover that the Nation is the oldest civilization known and the antagonists kill the character of Ataba.
The falling action is about the decisions of the Nation to not join the British empire.
The resolution/denouement comprises the revelation of the modern day Nation while we find things out about the main characters (such as the fact that Mau of the Nation died of old age and was given back to the ocean in order to become a dolphin).