Story structure
Subplot
A strand of the plot that runs as a supporting side story alongside the main plot; screenwriting's B story against the A story.
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." It differs from the main line in degree: less action, fewer significant events, less impact on the story's world, and it happens to less important characters. Screenwriting calls the pair the A story and the B story. The article's example is Mr. Robot: Angela Moss's first-season subplot, blackmailed into installing spyware while climbing Evil Corp, eventually feeds the main hack.
The craft risk is proportion. A subplot earns its pages by changing the main plot's terms, or by carrying the quieter register: the romance, or the secondary figure's character arc. Serial writers also use subplots structurally, alternating threads to pace a long run, which the ensemble cast guide covers from the cast side.