"Avatar" animated version generally stuck to just an A plot or a plot A and a plot B, with the gang in plot A, and Zuko and Iroh in plot B. Avatar Netflix's version has plot A, B, C.
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What's the difference between "a plot A" and "plot A"?

Question

"Avatar" animated version generally stuck to just an A plot or a plot A and a plot B, with the gang in plot A, and Zuko and Iroh in plot B. Avatar Netflix's version has plot A, B, C. 
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What's the difference between "a plot A" and "plot A"?

Answer

When it says "an A plot or a plot A" it is introducing the concept. Later, it uses "plot A" as a name, assuming you are comfortable with the concept already. But they both refer to the same thing.

Answer

The difference lies in specificity and how the terms are used:

1. "a plot A"
    * Refers to a generic example or concept of a "main plot."
    * It's less specific, implying that it's one of many possible plot structures.
    * Example: "A plot A usually focuses on the protagonist's journey."

2. "plot A"
    * Refers to a specific plot in the story being discussed.
    * It's more direct and assumes the reader knows which plot you're talking about.
    * Example: "Plot A in this episode focuses on Aang's training."

In your sentence, "plot A" works better because you're talking about a specific structure in Avatar.