What word best describes a minor or secondary story line in narrative story?

The best word to describe a minor or secondary story line in a narrative story is subplot.

Here's why:

* Subplot literally means "a subordinate or minor plot in a story." It's a common literary term that clearly conveys the relationship between the main story and the secondary one.

* Other possible words like "side story," "minor plot," or "secondary plot" are less precise and might be used in different contexts.

Example:

In Shakespeare's "Romeo and Juliet," the feud between the Montagues and Capulets is the main plot. The love story of Romeo and Juliet is the subplot, though it is a very important one.