I heard in an old interview of Patrick Rothfuss on the Writing Excuses Podcast where he mentioned how, after learning what they were, he couldn't stop seeing A plots and B plots everywhere. So, not knowing what they were myself, I of course had to look it up :)
Turns out, this is a neat way to look at your different story threads, or story arcs. For example, you could have a murder that needs to be solved (one thread), and the detective falls for the client (another story thread), all while dealing with his bestie's suddenly weird attitude (another mystery? Thread 3).
But instead of using numbers, we use letters.
The "A plot", therefore, is our main storyline, the one that will take up most of the book/movie. It follows your main character. In the example above, your detective needs to find out who killed the victim(s) and catch the murderer.
But having a story, especially a story that lasts a while (a novel, or an hour-long (or more) show/movie), be about only one thing, can be rather boring.
Enter the other plot lines (aka subplots).
The "B plot" is therefore another story happening within your Bigger Story, which may or may not directly tie into your main plotline (A Plot).
Using the same example as above, this is the plot line that follows your detective as he falls for the femme fatale character that came to ask for his help. That "romantic" thread would be considered secondary to the whole "whodunit" plot, but also be subservient to it.
If, on the other hand, the B plot is not linked to the main spiel, it must still somehow add depth to the story, albeit in more subdued tones than the A plot. This can be by:
- providing more background story into the main character (maybe he has another "side quest" where he needs to fix his relationship with his sibling, or his bestie),
- exploring the worlds of (appealing) side characters readers/viewers want to hang out with too, (including the Big Baddies themselves), or
- reinforcing the main theme(s) of the story (ex: how corruption is destroying everyone's lives, including our detective's, how justice will always prevail...or not, up to you to choose a resonant message).
Any additional plot line on top gets named according to the rest of the alphabet, in order of importance. And these, in turn, can help increase/maintain the tension in your book/script. Say your detective is having a pleasant moment with the paramour, the lull in that storyline can be filled with a thorny moment in one of the other plots (B/C/D...). Bonus points if these other plotlines somehow intersect the A Plot, and help solve it at the end.
The C plot (also known as "runner" in some circles) can also be a plot line that runs through multiple books/episodes, if you've got a series going on (hence its name).
In the Harry Potter series, the threat of Voldemort hangs over Harry and his friends the whole time, but in most books, the heroes' main goal is to deal with a more immediate need/threat (figure out what's turning the kids at school to stone, go through and survive the triwizard championship, etc.).
In terms of writing these other plot lines, it's crucial to understand that they each need to have their own arc too--a setup, mounting challenges, a climax (hopefully that echoes/reinforces the climax of the A Plot), and a resolution. This means that, although more plot lines can deepen your overall story, it necessarily makes it longer too...
And that's basically it! Have fun writing :)
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