A subplot is a secondary story that finds its way into the overall story arc, or the main plot of a novel.
The question is, what is a subplot good for? Better still, what qualifies—and what doesn’t—to be a subplot?
A subplot is a secondary story-line which is related to the main story (or plot line). It is used to expand, or deepen the character, to make your story more life-like (after all, very little in life happens in a single thread vacuum). The relation of the subplot to the main plot is through the theme.
The trick is to be sure that any subplot you concoct does indeed support and add to the main story theme.
If your subplot can be removed from the story without damaging the main theme, it does not belong there in the first place.
OK. Got it. A good subplot adds to the story, it expands the reader’s understanding of the characters and situations.
That’s good, right? So, if one subplot works so well, why not five? Or Eleven?
Too many subplots water down your story. They detract from your theme and message. They confuse. Two or three major subplots are about all that your story can sustain in a healthy way. More than that and you take the chance of confusing—and eventually—driving your reader off to another book.
Your homework: Take a look at three or four of your favorite novels. Identify the main theme, and then ferret out the supportive subplots.
Seeing how a pro does it will take much of the mystery out of the process.