Before I begin pontificating on how to plot a good mystery, I want to throw out this terrific warning. Go here and learn why getting into writing professionally may not be the best idea you ever had: http://whatever.scalzi.com/2009/11/09/writers-and-financial-woes-whats-going-on/
1. Think "What if . . . " What if you were watching TV some evening and your phone rings. Answering it with a mumble, you hear, "The blow will be in the green suitcase arriving on Flight 340 tomorrow at five." (What if you knew that “blow” is slang for powder cocaine?) What if a third grader overheard a teacher talking about murdering someone? What if you saw your best friend walk up to a squad car and hand the officer a large amount of cash? Every story has its origins in "What if . . . " You might also call this origin your SCENARIO. Catch the news, listen to gossip, read a sports magazine, watch a Discovery or Learning channel program with that in mind, and pretty soon you’ll have, like me, a criminal mind.
2. When you start to actually sit down to write your story, begin at the end. Nearly all mystery stories have a point near the end when the sleuth says, "Let me explain." He or she then reviews briefly what has been going on all along, concluding by unveiling the guilty party. Even if you do not plan to have a scene like this, you must have the solution as clear in your head as if you will. People studying to be police detectives are taught that in order to solve a crime you must answer, Who? What? Why? When? Where? and How? You should know the answers to these questions before writing your story. If you know where you’re going, it’s easier to get there.
3. Naturally, all this planning will invent the clues that will lead to the catching of your culprit. If you’re writing a “Play Fair” mystery, in which you will give your readers the opportunity to solve the mystery ahead of the sleuth, try breaking the clues into fragments dropped IN THE WRONG ORDER throughout the story. It can be clever to put your main clue in first, before your readers have your characters sorted out. (Make sure your clues are the sort your detective can discover and interpret. If the solution relies on esoteric knowledge about medicine, make your heroine a doctor. If the clues involve fingerprints, blood analysis, and the like, better make her a cop with access to a forensics expert. Or a forensics expert – but this will call for a LOT of research. If the clues involve the nuances of human behavior, don't make him a nerd or misanthropist, unless you also plan some major changes in his attitude.) Find some solid lead that points to a suspect (not the perpetrator) to drop in with calculated carelessness. Invent a clue that absolutely clears him/her -- and either mix it with a seemingly more important bit of information, or put it in BEFORE the clue that points. It is great fun to cause your reader to be certain Ingrid did it, when all the time it was the grandfather. BUT, the reader shouldn't feel cheated; she should say at the end, "Darn, I missed that clue, and it was right there in front of me." Read Agatha Christie to discover a mistress at this game.
4. Having all this sorted out and written down, go to the beginning and invent a "Grabber." You want an opening sentence that will draw your reader immediately into the story, or is so strange that more of the story simply has to be read. For example, "It was all Tom's fault; he's the one who brought an elephant to church." (For a “white elephant sale” he brings a white plaster elephant as his contribution.)
5. There can be any number of people mentioned in your story ("a cast of thousands" if you like) but your reader should be required to keep track of the names and actions of no more than seven characters. (You will need a victim, a detective, and least two suspects, so add new characters carefully.) Do not name them Don, Dan, Dave, Doug, Sue, Sandi, and Sally; name them Gloria, Herman, Ingrid, Jessye, Marvel-Ann, Pedro, and Desktop.
6. Keep the story moving, keep the reader guessing what will happen next, then toss in an ending that makes him laugh or feel surprised. But remember, the ending has to fit the story; don't cheat by making the perpetrator someone you introduce on the next to last page. On the other hand, some kinds of sleight of hand in short stories can be okay. For example: A little boy overhears a teacher saying something on the phone that seems to mean the teacher did something illegal. The teacher sees him listening and orders him to stay after school. The boy is so scared he appears sick, so another teacher sends him to the nurse, who decides to call his mother. The mother goes to talk to his teacher, who rises in confusion and stammers out something indicating he is guilty of a serious crime. Why? Because the boy's mother came straight from her job to pick up her son -- and she's a police officer! (See how the surprise comes because you don't know the mother is a cop until the very end? The boy in the story knows, and the nurse who called her knows. The trick here is to keep the reader from knowing until after the teacher has blurted something incriminating.) NOTE: this is not a play-fair mystery, but it's cute – and salable.
7. The hardest part of writing a story is the middle. New and young authors especially have trouble with this. They find (surprise!) it is actually work to write this part. They get discouraged and quit. Or they try to be clever and avoid the work by writing the beginning and then the end. This, unfortunately, leaves the reader with nothing to do -- such as get to know the characters, or solve the crime themselves. But the middle is where the story happens, so spend time on it. Why does your hero stick with it when all he gets is frustration? (Maybe the police think your hero murdered his wife, and the only way he can clear himself is to find out who really did it.) What do the cops think of his doing his own investigating? What does he think of doing it? How does he figure out where to begin? And what does he find out? Something shocking right away, so he -- and the reader -- understand he is right to start digging. Maybe he finds evidence in her checkbook of large, unexplained, regular payments -- was she being blackmailed? Maybe he comes across some dark secret from her past he uncovers when he is packing away her things: a baby, a stint at Betty Ford. Who had motive? Opportunity? Provide a lot of information -- too much. Is the fact that Jessye called in sick Monday a clue? Does the fact that Marvel-Ann came over to borrow a cup of flour on Wednesday when Ingrid says she bought a five-pound sack on Tuesday afternoon a clue or just a piece of information? Or is Ingrid lying? Where did Pedro get the money to bail out Herman? Does Desktop wear second-hand clothes because he likes them or because he can't afford to shop at Wal-Mart? Have fun, but remember, everything in the middle should move the story toward the solution.
8. Try to think what you would do in the same situation your put your characters in. And what your sister, or father, or Aunt Sarah would do. (Like dialing 911!) That way, your characters should behave like real people, and your story is easy for your reader to believe.
9. Trite but true: write what you know. But if you don't know, go find out! It is accuracy of detail that creates and sustains suspension of disbelief, and this goes double for mysteries, where "a fact that ain't so" is a clue. (Example: If a character tells the sleuth that she saw someone screwing a barrel-shaped thing onto the end of a six-shooter before firing it, and that’s why no one heard the shot, she’s lying. Silencers work poorly on revolvers.) Writers are researching all the time. They read everything. Some are adventurous, they explore, stretch, challenge themselves. Writers are curious; they never know what little detail will prove helpful next time they are building a plot.
10. Study the kind of stories you want to write. Read them several times. Try catching a favorite author as he or she lays down the plot, describes characters, slips clues in. You might even try writing a sample story in his style.
11. Keep Writing! The writer finds it both a love and an obsession, because only love (or obsession) will keep her sitting for hours and hours at her desk. If it is right for you, you will discover no pleasure in the world like that of getting lost in a world of your own making. You will come to care desperately about your hero, and wish him well even when you are putting him in a situation that will test him to the limit; you will find yourself angry at the villain, laughing when your hero is made ridiculous, smiling through your tears at a happy ending. (If this isn't happening, re-write until it does!)
12. Don't be discouraged by rejection slips; every writer gets them, even successful ones. Persevere: One day, possibly when you least expect it, you will get the immensely flattering news that a publisher wants to pay you for your story!
Although the rules set forth above are true and correct, so was Mr. Maugham when he said, “There are three rules for writing a novel. Unfortunately, no one knows what they are." Please don't forget that.