Subplots are just what their name indicates: plots which are under the main plot (Story A). Although there is a great deal of discussion about stories and their structure, far less is said about subplots. Why use subplots at all? Let’s examine the following story. Version 1: Girl moves to a small town and falls in love with a vampire boy. That’s it. That’s the only storyline we get. Version 2: We get an additional storyline. In that town, she runs into another boy, a werewolf, who tries to get her to love him. Version 3: We get a third storyline. The vampire boy is part of a vampire family, who try to stay disguised from the police chief and the other townsfolk. With version 3, the book, Twilight, now offers a richer fabric. The reader now gets more bang for her buck, having three stories rather than one. The story world also becomes more realistic, as more is happening. Our everyday world at any moment in time is surrounded with stories that interrelate: the stories of what is happening at work, at home, and with friends.
What is weak in many stories is the structure of subplots. An author will carefully plot out Story A, but will fail to do the same for the subplots: Stories B, C, and D. The structure of a subplot is the same as Story A. It should include the same narrative steps of: introduction; disturbance; obstacles; climax; and resolution. A subplot should have this story arc. The level of detail is less than in Story A, but having these narrative steps is still essential. A storyline will fail to engage without them. An author should ensure the volume of narrative devoted to any particular subplot is less than the volume of Story A, or run the risk of that subplot accidentally becoming Story A.
What ties a subplot to Story A? For instance, Sparky (a dog) lives with his owner, Gerald. One evening when Gerald is on a date, a thief breaks into their house and steals one of Gerald’s datasticks. Sparky knows the information on the datastick is sensitive, and follows the thief across town to rescue it. Meanwhile, another lesser story is told: about Muffins, a brown cat, who is doing everything in her power to learn to read. Could we get away with combining these two stories in one book? Not unless they connect somehow. Without a connection, the reader will consider Muffins irrelevant and be annoyed with her. Is it enough that Muffins lives in the same house as Sparky and is another pet of Gerald? That helps. The subplot about Muffins is tied to a character common to both storylines. But without a better connection, the reader may still end the book wondering why Muffins was included. Was she just filler, a tool for getting a larger page count to the book?
Perhaps Muffins has a laid-back personality, while Sparky is very energetic. Better. The two pets serve as foils for each other. Try this: in the climactic scene, Sparky has chased the thief onto the rooftop of a building, where Sparky then finds himself hanging from it, the thief ready to step on his paws. Suddenly, Muffins jumps onto the thief’s back, piercing his skin, giving Sparky a new burst of adrenaline and time to climb again onto the roof. While the thief fights with Muffins, Sparky snatches the datastick, and both pets run away. Now Muffins is relevant! Muffins changed the direction of Story A: she reversed Sparky’s fate and reversed whether he would achieve his story goal. The principle, then: a subplot should change the direction of Story A.