Write a story in which your principal character has a dirty secret, something that they are ashamed to tell anyone else.
They are hoping to get away with it, but will they?
Add a plot complication in which someone else finds out. They want something in return for keeping the secret, "Your secret's safe with me."
But, it isn't really, is it? Or, maybe, your protagonist can't take that chance, so they take precautions, or drastic actions to protect themselves... and that's when things get out of hand.