A novel with only one story is too simplistic and therefore, plain and uninteresting. Moreover, it would be difficult to believe, as total isolation of events in real life is not frequent; the appearance of secondary characters implies secondary stories. It is necessary to control these sub-plots so that they do not get out of our hands or start telling by themselves stories that have little to do with the main theme (parasite stories). Sub-plots can reinforce the theme of the main plot, contradict it by adding another point of view, or generate secondary themes that will bring color and life to the environment of our story.
Each sub-plot shall have its story, its theme, its premise, its protagonist (one or several, which are generally secondary characters to the main plot), its plot and its resolution. Due to the way in which they affect the main plot, sub-plots can be collaborative (necessary, complementary, or opposed) or parallel. The first ones are minor plots that help in some way to the resolution of the main story, either by solving a necessary aspect, clarifying the action of a character, or providing some logical cause to a situation lived by the protagonist. Parallel sub-plots do not help the main plot, but they provide in some way different points of view on the subject-matter and they enrich the treatment we, as writers, want to give to the story. It is very important for parallel sub-plots not too difficult or dissolve attention on the advancement of the main plot. If that should be the case, it would be better to reduce them or simply cut them out of the novel. They may be a good idea for a future novel.
Once sub-plots are resolved, we will begin to organize their events inserting them in the middle of the main plot, without forgetting the principle of cause-effect and taking care that their inclusion is not affecting other sub-plots. This is a craftsmanship writing work, where our expertise in the creation of complex, yet coherent and believable novels, is shown.
The result of this mix of plot and sub-plots is finally the argument of our novel.