Mysteries and especially their first cousins, the thrillers can be full of action and adventure (external stories) but at heart both are journeys – journeys of the mind (internal stories). If done well, they are journeys as much for the reader as they are for the protagonist.
Read the greats: Agatha Christie, Arthur Conan-Doyle, Dorothy Sayers, Dashiell Hammett and you will quickly see what I mean. From the beginning of the riddle to the revelation at the end there are miles to go before you sleep. True, National Treasure may be seen as a thriller or mystery as much as a quest. In The Hunt for Red October, action may be the draw. It certainly is for an author like Ludlum. Yet like any mystery, when the clues are followed, the mystery slowly unravels.
So the protagonist starts with a puzzle, perhaps something like a great jigsaw puzzle, and has to put the pieces in just the right places to see the final picture. The journey, then, is from ignorance to knowledge, from confusion to understanding. From questions to solutions.
As with any journey plot, the trigger comes quick. I would not recommend a chapter on what a wonderful person the victim is and another chapter on what an insane, evil creature the murderer is. There is a reason why so many books start with a dead body. That is where the mystery (the journey) begins.
Mrs. Lavender kept being slapped in the face by her own scarf as the wind roared through the broken conservatory window. She did not mind, however, since she was dead. The kitchen knife was planted firmly in her chest…
Professor Pinch was lying on the plush oriental rug in the library, but he was not taking a nap. The lead pipe with the blood stains beside his head assured that he would never take a nap again…
Colonel Ketchup’s body swung from the end of the rope. The chair was turned over and one of the officers handed his superior the suicide note in a plastic bag. It looked like suicide, but as the chief detective reached for his Tums he decided it smelled like murder.
Once the protagonist enters the picture, it is off to the races. There will be obstacles throughout the middle of the story like any journey plot and getting lost (misdirection toward the wrong suspect) is almost expected. In the case of the mystery or thriller, though, there is another element that needs special attention: the clues.
The clues, above all, make for a good mystery and the slim chance that a reader might figure it out keeps the reader to task. These must be done with great skill and dexterity, and probably why I will never be a true mystery writer. I am too blunt. But when done well, they make perfect sense at the end. No one should doubt if the Butler really did it.
Again, as with all journey plots, the end comes with success or failure. We are accustomed to success (probably because of all those detective/police dramas on television over all of those years). But sometimes the antagonist gets away with it. Everyone, including the reader knows, but… The question in that case is should we be mad (upset) or cheer that they got away with it?