Tell me the plot of "The Hound of the Baskervilles."
Easy enough, you say. A country doctor comes to Sherlock Holmes and Watson. The local lord has died of heart failure. But there were a giant hound's tracks near his body, and there's this family legend about....
But is that the only plot?
Not really. Long before old Sir Charles is frightened to death by a hound, there is a man in South America, a distant relative of Sir Charles, who decides he will be the new lord of Baskerville Hall so he changes his name, makes his wife pretend to be his sister, and....
Some mystery writers call this second storyline the back plot. It is the story behind the story. The detective's plot is the discovery of the back plot. Holmes must reconstruct the murderer's back plot through the clues left behind. He must understand what happened before.
This twining together of two plots is the glory of the mystery and the agony of the mystery writer for she must not only have one plot which is logical and interesting. She must also create a second which intersects it backwards in time.
No, that's not crazy. Think about it. A murder occurs, and the detective investigates. He finds clues, and these clues point toward the past of the victim and the murderer. The detective must decipher these clues to discover the who, what, when, where, and why of the murder. He must travel back in time to the murderer and his motives.
Holmes studies the crime scene, the stories of the butterfly collector, the sounds of the moor, and the ancestor's portrait, as well as other clues, to find that distant Baskerville relative who has designs on the family fortune.
How does a writer create these two plot lines? The answer to that is as diverse as the authors questioned. Some create the back story, pick the relevant clues to pepper the novel with then set their detective to work.
Other writers are as surprised as their detective at the murder scene and never guess the killer until the last chapter. Somehow the clues, through the miracle of the writer's subconscious and a little judicious rewriting, have pointed to the murderer all along.
Still other writers mix a little of both methods. Cold calculations about clues and the killer's identity are leavened by the spontaneous generosity of the writer's muse. The writer is as surprised as the reader to discover why the killer hums but never sings and how that fits so perfectly into the puzzle.
No one can tell you what method to use to create a perfect blend of detective's plot and back plot. Each writer must discover what works best for her. But the wise writer takes the time after the book is written and before the rewriting to ask herself, "What is the plot? Does it make sense? Is it complete?"
Then the even wiser writer asks the same questions about the back plot. The wisest writer also remembers that in the back plot the killer is the major protagonist, and it here where the true heart of the novel lies.
Now tell me the plot of "The Hound of the Baskervilles."