So far, we've discussed how you start with a germinal idea and what makes a good one; then we discussed the villain profile and how to create a villain with a dark mission, who will take actions to get what he wants. These actions are the plot behind the plot. The hero's job in a thriller, remember, is to foil evil. The villain's plot behind the plot is the evil the hero must foil. These actions the hero and others take to counter the plot behind the plot make up the plot of your thriller. Simple, no?