This article, from Laura Backes, originally appeared in the February 2010 issue of the Children’s Book Insider newsletter and is reprinted here in its entirety with the CBI’s permission. While the CBI newsletter is primarily aimed at authors of children’s books, the advice given here is applicable to any work of fiction.
To add texture and depth to the main action plot. Sub-plots simply make a story more interesting. Where would Charlotte’s Web be without Templeton the rat, and his evolution from self-centered, gluttonous scavenger to heroic, gluttonous ally? Some sub-plots are small diversions (think of Fern’s spending less time in the barnyard as she gets older), others crucial to the progagonist resolving his problem. Sub-plots offer the author another opportunity to throw obstacles in the main character’s path (by distracting the protagonist, making his life more difficult, or introducing characters with their own competing agendas), or give the protagonist tools to make his life easier.
To press the "pause" button. Sometimes, a story may be so tension-filled or stressful that the reader simply needs a break. Sub-plots keep the reader involved in the characters’ lives but offer a rest from the action. Conversely, if the main story is quiet and thoughtful, a more action-filled sub-plot helps vary the pacing. Be sure your sub-plots always develop elements of character or story that give the reader new information that relates to the primary story arc. Plot tangents that dead end, rather than loop around and eventually come back to the central story, are pointless.
To illustrate your theme. Sub-plots are often emotionally-based, exploring relationships or internal aspects of your protagonist. Because of this, a story’s theme is often revealed in the sub-plots. If the main plot of Charlotte’s Web is whether or not Wilbur will die, then the friendship between Charlotte and Wilbur is a primary theme. This friendship ultimately saves Wilbur’s life. Because E.B. White illustrated the power of friendship through a complex sub-plot, he showed us the theme. He never had to tell us what his message might be.
Another way to show theme is through the growth and change of your protagonist. Sub-plots are a vehicle for this as well. Wilbur grew from an timid, hysterical, lonely piglet who lacked self-confidence to a radient, resourceful, loyal pig. Elements from every sub-plot in the book contributed to his transformation.
Sub-plots don’t necessarily end with a firm resolution as your action storyline will. Sub-plots give your character skills and experiences that he’ll take with him beyond the last page of the book, so they can be more open-ended. For upper middle grade and young adult audiences, some sub-plots might end with the death of a loved one, or a friendship ending. But reserve a few hopeful threads for the final pages that show the protagonist is moving in the right direction. The most satisfying stories give a balance, as in real life.
If you’re an author of children’s books, or aspire to be, take a look at the Children’s Book Insider Clubhouse site. There, you can register to receive the monthly CBI newsletter, which is filled with more useful articles like this one, as well as notices of publishers and agents seeking children’s book manuscripts.