A subplot is a secondary chain of events that occurs in a fictional account and serves to broaden the scope of the main story or plot and the details revealed about the characters. In fiction, a subplot is often thought of as a story-within-a-story, and one of the main reasons for including it is to expand the scope of the main story so that it can have a depth more closely reminiscent of the complexity of reality itself. Often, a subplot is used to expand upon the motivations of supporting characters that are either working for or against the main characters, or, by their actions, are setting into motion a chain of events with which the main characters must somehow deal. Despite the fact that subplots and their supporting characters are of secondary importance to a story, they must also have a beginning, a climax, and an ending in some fashion so that the reader is not left wondering what the purpose was for their existence in the first place.
The use of a subplot in a story usually has one of two effects on how the story is told. In most cases, it is tied directly to the main events in the story and occurs in the same general time period. In this way, it has an immediate effect on the characters and situations that arise as the story unfolds. An alternative approach to using subplots is to have them run parallel to a story, but independent of its main events, for the most part, to serve as a form of contrast that better illustrates the decisions that the main characters are making. In elaborate narrative accounts, one subplot may also feed into another before the main plot is affected in a layered effect that is often used in mystery, political, or military fiction to slowly reveal the nature of what is really taking place in the story on a grander and more encompassing scale.
Often, a secondary plot strand is necessary to explain the hidden motivations behind why a character acts the way that he or she does. A heroic character may, for instance, demonstrate bravery in most situations, but cower in the face of something unusual that takes place in the main plot. In order to keep the story true to its direction, the motivation can be explained in a subplot that reveals hidden traits about the character's past.
Subplots are usually employed to create justification for unexpected events and actions taken by the antagonist and protagonist in the story so that, when the story concludes, everything that took place appears to have happened for a good reason. This is one of the key differences between fiction and real life, where characters in the real world often make decisions that are hard to understand on the surface. Supporting plots also add an element of reminiscing or back story to the main plot without slowing down the action directly. This is a way to illustrate the internal reflection and memories that the main characters may have to give them a more three-dimensional nature.
While the subplot is common to novels and long works of fiction that involve many characters, such as in screenwriting, such literary concepts are often not used in short stories. This is because short stories usually only have a small handful of characters and one central event around which all of their behavior and attention is focused. The main point of a subplot is to provide contrast for the events of the primary story so that it can become more intricate and meaningful, which goes against the rapid climax of a one-act story arc in a short work of fiction. Longer works of fiction, however, usually have up to three subplots that are tied into each other or the main story in some form.