We think the story is about Black trying to kill Harry, so the plot seems focused on that; but that's not what the story is about.
In a way, it’s true that there is no single central plot in Prisoner of Azkaban, because one candidate (Quidditch) lacks gravitas and another (Sirius v. Harry) proves to be an illusion. But in terms of what plotline drives the book, I would say it’s the latter. We “know” from very early on that Sirius Black is trying to kill Harry, and we know there will be a confrontation at the end—and there is. The only reason that we look back and say “that wasn’t the main storyline” is that there’s a twist. And that’s why the story doesn’t wrap up in the Shrieking Shack, even though that scene seems like it’s going to be the climax.
Beyond that, there is another focal point: the whole backstory of MWPP (and S). One of the many things I love about the book is that while Harry is going about his life—lighter things like wanting to go to Hogsmeade and playing Quidditch, heavier things like hearing his parents and coping with Dementors–there is another drama mostly invisible to him (and to us, until the second reading): that of Lupin, Black, Snape, and, if you think about it, Pettigrew.
We think the story is about Black trying to kill Harry, so the plot seems focused on that; but that’s not what the story is about. It’s about Sirius in a whole different way, and it’s as much about Pettigrew, and right on out of the pages of this volume to Voldemort.
The more I think about the plotting of PA, the more impressive it seems. JKR has all these threads going, and they’re interwoven in amazing ways:
- the most-important-to-Harry thread (Quidditch)
- the emotional thread (Dementors/J&L). It is interwoven with Quidditch, without which Harry wouldn’t keep hearing his parents’ voices.
- the drives-the-plot thread (Sirius trying to kill Harry), interwoven with the seemingly trivial thread of Harry trying to get into the village (key because of the Sirius plot, and because it introduces the Map)
- the true Sirius story (evidence throughout that Sirius is trying to save Harry—also, things like the Firebolt, brilliantly tied to the Quidditch thread but also establishing Sirius’s character and the Sirius-Harry relationship, once we learn the truth)
- the false and true Crookshanks/Scabbers/Pettigrew storylines
- Hermione’s schedule (and accompanying crabbiness), which is a minor and humorous storyline but becomes central to the plot by the end
- the Buckbeak thread, which also seems to be mostly about Hagrid and Draco but becomes central by the end
- the character of Trelawney and Divination—all the set-up about whether to take Divination seriously
- the character of Lupin—which is probably a lot of setting-up for stories yet to be told in OP and (knock wood) beyond.
- the character of Snape, who hits a low in this book but whose backstory is also set up for the revelations of GF, especially poignantly if he turns out post-GF to have been the spy who tipped off J&L.
Everything balances. The storylines that seem trivial either turn out to be central (Crookshanks v. Scabbers) or serve to bring in storylines that are essential (Quidditch, e.g., brings in Sirius-as-godfather, and the Dementors/J&L issue). The real character dramas are largely below the surface (interactions among the MWPP generation). It’s amazing.