A subplot is a smaller story within the main plot.
Every subplot is a story with a protagonist and an antagonist (not necessarily the protagonist and antagonist of the main plot) who are in a struggle to achieve their goals. The difference between the main plot and the subplot is that the main plot is the story that the reader invests in; the subplot is a shorter story that supports and adds layers to the main plot.
One way subplots serve the main plot is in echoing its theme or conflict; another is serving as a foil to the main plot by contrasting with it. But a subplot has to do more than subtext, it also has to materially influence the main plot.
Subplots should be plotted in the same way as the main plot with the subplot scenes (usually those scenes with the subplot protagonist as the POV character) distributed evenly across the acts, beginning after the main plot has been introduced and ending before the main plot ends, often in the third act as the subplot merges completely with the main plot. (See Merging Plot and Subplot on Argh Ink for more explanation.)
EXAMPLE: Remember Jane, the tightrope walker who wanted to be queen of the circus but was attacked by bats, a Nopefish, and a bomb? That was the main plot:
The Nopefish’s subplot echoes Jane’s plot in theme: they both let fear hold them back. It also contrasts with Jane’s plot because Jane works to overcome her fear while the Nopefish lets his fear rule him. But the most important thing about the Nopefish’s subplot is how integral it is to Jane’s plot. It’s Jane’s story, but Jane’s story would be impossible without the Nopefish’s secondary story.
A subplot’s first purpose is to serve the main plot, but it should be a compelling story on its own.
If your story seems overlong and unfocused, try taking your subplots out one at a time and looking at your story as a whole again. Any subplot that can be removed from the story without affecting the main plot should be cut; your story will feel more unified without it.
For other posts in this unit, see The Structure Unit Table of Contents.