What's happening here is the word narrative, which is normally a noun, is being used as an adjective, allowing the writer to introduce another noun arc which that adjective can modify. It's actually a fairly common term in Lit Crit, and this is as good a definition as any...
narrative arc refers to the chronological construction of plot in a novel or story.
It's probably being used somewhat loosely in OP's context to mean that the author has a consistent overall approach when writing about all nine parallel universes, and/or he's careful to ensure "seamless continuity" (using segues, perhaps) as he moves from writing about one to writing about the next.
To be honest, I don't think it would make much difference here if the word arc were to be simply omitted.