サブプロット

Also known as B plot, B story, C plot, C story, secondary plot

In fiction, a subplot or side story is a strand of the plot that is a supporting side story for any story or for the main plot. Subplots may connect to main plots, in either time and place or thematic significance. Subplots often involve supporting characters, those besides the protagonist or antagonist. Subplots may also intertwine with the main plot at some point in a story.