A plot line is basically a story line in a book. It has an arc and must cross multiple chapters. It doesn’t have to appear in every scene, and–with the exception of the BIG plot–it does not have to start in the first chapter and get wrapped up in the last.
How many plot lines should you have? Honestly, I think it depends on your book, but here are the ones I always have:
1) The BIG plot. Every book needs a plot line in which all of the characters are interested. This is the mystery that keeps the characters (and readers) hooked for the whole book. For example, who shot Jim?
2) The small plot. This is another plot line that affects everyone in the book, but this one is less important and perhaps more trivial. It can be used for the times when you need the MC to forget about the BIG plot. It also helps provide more depth to the book so it doesn’t seem like everyone is literally obsessed with the BIG plot. For example, everyone wants to find out who shot Jim but they are also puzzled by the disappearance of Bob’s dog. In the end, these plot lines will intersect and we will discover that Bob’s dog ran away when the murderer accidentally let him out.
3) The B story. This is the secondary plot and in many books, it’s the romance. For example, will Sue and Tony get together? This plot line affects two (or more) characters but not everyone in the book. It usually starts at the beginning of Act 2 and wraps up before the ending of the BIG plot. Why? Because it’s better to nest your plots (meaning, wrap up the minor ones before the major ones).
4) The back story plot.This is the “what really happened to get the MC here” story. So if your MC is trying to solve the murder, you can have a sub-plot about why she got the case in the first place. The point of this plot line is to give the MC’s back story with a purpose. Sure, you can just tell the reader what happened to her when she was 12, 13, etc…, but it will be more interesting if you make the reader WANT the back story.
5) The MC’s outer goal. This is the thing the MC is trying to achieve in the book. For example, get a promotion. Other characters will influence this plot line (especially the antagonist who is the main person trying to thwart it!)
6) The MC’s inner goal. This is what is referred to as the character arc and it is THE MOST IMPORTANT PART OF THE BOOK! This is the thing the MC wants internally. For example, to finally feel accepted by her father. In certain genres (like YA and Women’s fiction) people read for the character arc. If you don’t have this, the book is blah.
7+) The secondary characters’ arcs. If you have secondary characters, they will have much more depth if you give them their own arcs. For example, the best friend really wants a job as a hand model. In my last book, I had these for each secondary character.
That brings us to 10 (for me). This may seem like a lot, but if you map them out in a graph with the key points noted for each Act, you will find it’s not hard to use this many at all. Well, not until your agent tells you to change #2 and the whole thing falls like a deck of cards. Not that I would know…*rushes to clean up cards on floor*…*just kidding*.