An embedded mystery, or a mystery inserted into a larger plot, poses a unique challenge to its creator, particularly when the narrative is clearly moving in a certain direction and the mystery slows it down. How do I make this sufficiently complex and interesting without it seeming like a way to stall? Do I use existing characters, or make new ones? How do I make it part of the narrative and not just a slightly more cerebral Big-Lipped Alligator Moment? In short, how do I maintain continuity?
One useful source of plot continuity in an embedded mystery is influence by and on the overarching plot. Mystery interrupting your metaplot? Make it part of your metaplot. Perhaps some element of it is clearly caused or influenced by that which came before; it’s vengeance for an earlier slight, people come to the group because they know they’ve handled crazy things like this before, you get the idea. And for the retrospective crowd, make the mystery feed into later events; the crime might pave the way for a later plot, someone who was Hanging Around Mysteriously but not actually the culprit may be explained later, and so on. This doesn’t just have to be events; knowledge can be both utilized and foreshadowed just as cleanly. In short, if you want to make sure a mystery doesn’t seem like a break from the plot, connect it to the plot.
Continuity of context also helps to keep the plot connected. While a few new characters make the mystery more interesting, it’s better not to improvise the entire cast; why create an entirely new set of characters when you have perfectly good ones already? Using people and places you’ve already introduced keeps similarity with the prior storyline, further tying the mystery to its context. On top of that, it gives added advantages; familiar characters come with preconceptions to play a red herring off of, reasons why witnesses would trust their investigators or experts would help them, and a greater sense of attachment (or shock, if a friendly character is the one who did it) to the plot. What’s not to like?
Can you explain why the mystery is sitting there in the middle of the larger plot, beyond “I felt like a change of pace”? If so, good. Serving a purpose in the storyline makes a mystery—or any other plot, really—seem necessary, and being necessary keeps it from being seen as filler or a stalling tactic. Like any other subplot, a mystery might provide an otherwise neglected character with history or a spotlight, advance an emotional element; the possibilities are pretty wide.
Properly embedding a mystery can be difficult, but it’s good for protection against complaints. How do you keep the story flowing?