The main secondary storyline in your narrative, usually carrying the emotional or thematic heart of the story.
The B-story is the primary subplot running alongside your main plot (the A-story). While the A-story usually handles the external conflict, the B-story often carries the emotional weight, whether that is a romance, a friendship, a mentor relationship, or an internal struggle. It is not just a side quest. The B-story is the storyline that teaches your protagonist the lesson they need in order to win (or lose) the main conflict. In screenwriting frameworks like Save the Cat, the B-story typically begins right after the break into Act 2.
The B-story is where theme lives. Your A-story asks the dramatic question (will the hero save the day?) but your B-story answers the thematic question (what does the hero need to learn about themselves?). Without it, you get a plot that moves but never means anything. The B-story also provides natural pacing relief, giving your reader room to breathe between the high-intensity beats of the main conflict.
The A-story is Elle getting into Harvard Law. The B-story is her friendships with Paulette and Emmett, which teach her that her real strength was never about impressing Warner. It was about being herself.
The A-story is survival in the arena. The B-story is the complicated relationship with Peeta, which forces Katniss to grapple with performance versus genuine human connection.
The A-story is Walter White's meth empire. The B-story is his disintegrating family life, which acts as the moral thermometer for how far gone he truly is.
Your B-story should connect to the A-story thematically. Ask yourself: what does the protagonist learn in the B-story that they need for the A-story climax? If the answer is nothing, rethink the B-story.
The B-story should begin early in Act 2. If you introduce it at the midpoint or later, it will not have enough room to develop and its payoff will feel rushed.
The B-story should be the harmony, not the melody. If readers are more invested in the subplot than the main plot, consider whether your A-story has strong enough stakes.
The B-story needs its own resolution, ideally converging with the A-story climax. When the protagonist uses what they learned in the B-story to solve the A-story problem, that is the magic moment.
Identify the A-story in something you are writing and state its central conflict in one sentence. Now write a scene introducing a B-story character who embodies your theme from a completely different angle. Show the protagonist learning something in this scene that they could not learn from the A-story alone. Finally, sketch how this B-story lesson will become essential at the A-story climax.