The elements of a good mystery are usually those of good atmosphere and suspense BUT if the underlying structure of the mystery is weak, all the adjectives in the world won't fix it. I have included here for your perusal a list of items that I use to help me form the underlying structure to a plot. Please enjoy.

Who is your antagonist?

Think like them. Remember they may not be privy to as much information as you, the writer, are so try to remember what they know and what they don't know. Have them work through a plan to commit the crime, then think about what the consequences of their action or inaction may be. How did they feel about what they were doing? Could they have been so nervous they overlooked something? Could they have been so casual that they assumed they would get away with it and had become careless? Could they have enjoyed doing what they did so much, they took a souvenir?

Exercise:
Plan a perfect murder/theft and write it down double-spaced. Do not think of your protagonist at all, only the antagonist. Place the work in an envelope and put it away for one whole week. Open the envelope and review the murder/theft. Where did you/could you have gone wrong had you tried to carry it out? Write down in the margins your errors. These errors are the clues that will guide the protagonist towards the truth. Analyze these flaws and why you made or might have made them. The answers are the flaws of an antagonist that will be the puzzle pieces found by the protagonist. If you honestly can't find any flaws with your plan, ask a friend to read over it and find if they can.

Don't forget your supporting cast
A good mystery can be ruined by process of elimination. "Well, it's not Johnny, so it must be Sue.. She's the only other character." Remember, give enough of a cast to your mystery keep them guessing. Why wouldn't Dr. Billingsly have committed the murder of his wife, Susan? He stood to gain from insurance and they had been fighting. What wasn't it her brother Ted? He's still mad at being left out of their father's will and Susan squandering the money on her gambling debts. Could it have been the lawyer who had been blackmailing her with risque videos she made with her ex-boyfriend. Could he have been so needy for money that when she refused, he went mad and killed her? In a nutshell, leave your options open.

Make a timeline of events
What happened and when? Make sure you make a timeline for not only the antagonist's crime, but also for the order in which the protagonist discovers the clues. If you rush the discovery of clues, you will give away the ending prematurely which may lessen the reader's interest. If you hold too many clues back until the end, the reader will not see it coming and might be disgusted at what they may consider a strange turn. "Hey, that's unfair. I didn't get to review that clue.", they might say. Some clues you keep close to the vest but if they don't have a majority of the clues, they will feel left out. Part of the thrill of a mystery is seeing if you can guess who it is before the ending.

Example:
9:30: Dr. Billingsly gets home but does not see Ted exit through the door he just opened. He chalks it up to the delivery boy.

9:45: Dr. Billingsly finally discovers the body of his wife, shot dead through the heart. He rushes to her side to see if he can resuscitate her (this is how he gets her blood on him.) He pries the gun from her hand (planted by Ted to make it look like a suicide) and the gun goes off into her leg as it was left on hair-trigger and Dr. Billingsly is not familiar with guns.)

Note: this is how he gets her blood, powder residue and his fingerprints on the gun. Forensics report later shows the fingerprints were at an odd angle to fire the gun and the second shot was obviously from her laying down and having been dead.

9:50: Dr. Billingsly gathers himself and makes the call to 911.

Questions to ask along the way
when making a timeline
Who (the antagonist) did what (the crime) and why (motive)? Did the antagonist try to leave misleading clues and, if so, how can these clues be discounted as being false by the protagonist? What was their critical error(s) that made it possible for them to be connected with their deed? Did the antagonist's plans go off without a hitch? How do these pieces point to the antagonist and where might the pieces lead elsewhere? Where does the protagonist get involved in the timeline of events? In what order does the protagonist gain the clues necessary to conclude the antagonist's guilt? When does the protagonist begin to put the pieces together? Why do the all the pieces once put together not lead to the assumption of someone else's guilt?

Make a Suspicion map
On your timeline, make notes of who you think the reader and protagonist suspect when and why. I also make little index cards with clues on them (I call 'em Clue Cards), so I can move them around the timeline to make sure they fall in the order that I want to. Remember, the right clues at the wrong time might mislead the reader or protagonist. This is not necessarily a bad thing but something that does need to be monitored and dealt with.

Exercise:
On a word processor, take a narrow left-handed margin. (The reason for this is so that it can be modified easily later but, if you wish, a legal pad with a column on the left will do just fine). Do your timeline for your story in that margin. Print it out and in the right hand column, do your Suspicion map. You may want to also use the Clue Card method mentioned earlier to move your discovery of clues around the timeline. Double-check your structure to make sure the discovery of clues leads your protagonist/reader where you want them to go.

Ask yourself questions
It is best to have an unprejudiced observer in a friend at this point. Sometimes we can get too close to the story and a few simple questions may solve the mystery outright. You need to explain some of the answers, in the story. So why didn't the protagonist ask them?

Roger: "Say, Bill. Why doesn't Jupiter Girl use her telepathy to solve the mystery? She can just walk around reading people's minds 'til she finds the bad guy and blammo, whack 'em on the spot."

Bill: "Er. uh. its during a vernal equinox and her telepathy doesn't work because of sunspots."

Roger: "Er. yeah. doesn't mention that. and that's lame."

Bill: "Well. um. how about its against the law in that world and she's a legal kinda gal."

Roger: "Sounds good."

Bill: "I'll write that in later."

Roger: "Cool. Taco Bell?"

Bill: "Sure. You're driving."

If you can be your own sounding board, this is great but I rely upon friends to help me. When I can't, I use a trick I learned from painting. I put the thing down for an hour and don't even look at it. Then I go back in and observe it closely looking for all the flaws. I list the flaws, and only then do I begin to work on them. If you work on the first flaw you find, you may not find the others without putting down the work for another hour. Time is precious to me so I try to get it all out of the way at once.

In closing
Everyone wants to read a good engrossing story. The things I have presented here are certainly not the ONLY way to do things and doing them will not promise that other story elements (pacing, atmosphere, drama, suspense, etc.) will fall into place. You may use all or none or one and not the other but what matters is, do what feels right and what you feel helps you the most. Think of these exercises and suggestions as tools. Every good craftsman knows when to use tools and when NOT to use tools.

Sincerely,
Robert Kemp