If the MC is keeping a big secret of some kind, there are certain things I expect to see, as a reader (or viewer, in the case of movies). The secret in question has to be BIG, meaning it's a vital part of the main plot. Better yet would be that it IS the plot. For example, the MC pretends to be someone she isn't, for whatever reason that serves her purpose in the story conflict.
Secrets are highly effective way to create tension and conflict, and personally they're one of my most favorite story elements.
If you're writing a story like this, here are some things to keep in mind regarding structure. The main turning points will show how the secret evolves during the course of the story.
Please note: the following is a general guideline that can be applied to any genre. Seriously. Anything. Sci-fi, fantasy, contemporary, romance, historical, thriller... It's the details of your story that will make it unique. Don't fear the standardization of a specific structure. It keeps the plot woven tightly together and focused, and delivers your readers' expectations.
Opening -- MC's life before the secret
Catalyst -- MC presented with a secret she can keep that might help her
Debate -- MC weighs the options and decides keeping the secret is a good idea
Act Two, first half -- MC's immediate benefits from keeping the secret
Midpoint -- MC realizes, despite the benefits, there is damage being caused by keeping the secret, either externally, internally (guilt), or both; realizes she made a mistake in keeping the secret, but feels she's "in too deep" now to change it; MC may also realize that she can't keep this going forever...
Act Two, second half -- MC's secret inadvertently discovered by a supporting character (either on her side or on the antagonist's side), but not the main person the MC is keeping the secret from; damage caused becomes more prevalent, and compounded by the other character knowing things she shouldn't; the MC may try several times to reveal the secret on her own, but can't get herself to do it, or things keep preventing her from doing it
All Is Lost -- MC's secret somehow revealed to the main person, or group, she was keeping the secret from, in the worst possible way; MC now feels completely out of control of her own fate
I want to stop here for a moment because the All Is Lost moment is so vital in this type of story, and I've seen it done poorly too many times.
This is the most important part of the post:
If your MC is keeping a big, bad secret from someone, the absolute worst thing that can happen, in her eyes, is to have that person discover her secret. And the longer she's been keeping the secret, the more painful it is upon the other person discovering it, for both of them. So to have a strongly effective All Is Lost moment in the Secret Plot, this is when the secret must be revealed to the exact person (or group) the MC does NOT want to know about it.
By doing so, your MC will be understandably thrust into the depression of the Dark Night of the Soul that results in making a firm decision to fix everything. That won't happen believably if you don't have your Big Reveal at the right moment, and then your entire ending falls flat.
Please trust me, as a reader. These are my favorite types of stories to read, but if the Big Reveal isn't in the right place, I may just throw your book against the wall. It's that frustrating.
Dark Night of the Soul -- MC realizes just how badly she f*cked up, but with the burden of the secret no longer weighing on her she can now make a plan to fix it all
Act Three/Finale -- MC fixes the very problem she herself created in the beginning
Ta da! Easy peasy lemony squeezy.
Can you think of any stories that follow the structure of the Secret Plot? There are SO many. I'd like to see which ones stood out to you!