A corpse. A locked room. No way in or out.
The locked room mystery is the purest distillation of detective fiction-an impossible crime that demands a logical solution. It’s also a covenant between author and reader more sacred than any other genre.
This is fair-play detective fiction, where the writer makes an implicit promise: You have all the clues you need. The solution is possible. I am not cheating.
Break that promise, and your reader will never forgive you.
The Golden Age and the Rules
In the 1920s-30s, during the Golden Age of Detective Fiction, writers took this covenant so seriously they created explicit rules.
Ronald Knox’s Decalogue (1929)
Knox, a Catholic priest and mystery writer, laid down ten commandments:
- The criminal must be mentioned early
- No supernatural solutions
- No secret rooms or passages (unless clearly established)
- No undiscovered poisons or impossible science
- No “Chinaman” (racist language aside, this meant: no mysterious foreigners as deus ex machina)
- No intuition or accident solving the case-only logic
- The detective can’t commit the crime
- The detective must share all discovered clues with the reader
- The sidekick must not hide their thoughts if they’re a POV character
- No twin brothers or doubles unless previously introduced
These rules exist for one reason: maintain reader trust.
Van Dine’s Twenty Rules (1928)
S.S. Van Dine (Willard Huntington Wright) went even further with twenty rules, including:
- The reader and detective must have equal opportunity to solve
- No love interest to distract with romantic subplots
- No conspiracies or gangs (too hard to deduce)
- The culprit must be a significant character
- The solution must be logical, never psychologically explained away
Some of these are dated (modern readers tolerate romance and psychology), but the core principle endures:
The reader must be able to solve this.
What Makes a Mystery “Fair-Play”
1. All clues are presented
The reader sees everything the detective sees. No hidden evidence that’s only revealed during the solution.
Example: Agatha Christie’s Murder on the Orient Express
Every passenger is interviewed. Every alibi is checked. All the physical evidence is described. When Poirot reveals the solution, you can flip back and see that yes, the clues were there.
2. The solution is logical
No intuition. No lucky guesses. No psychic visions. The detective (and reader) must be able to deduce the answer from the evidence.
Sherlock Holmes’ principle: “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.”
3. No cheating
The writer doesn’t withhold information, introduce new capabilities at the last minute, or use coincidence to solve the crime.
What constitutes cheating:
- The detective had a crucial conversation we never see
- A character turns out to be a master of disguise we never knew about
- The solution requires specialized knowledge the reader couldn’t possibly have
- “It was all a dream/hallucination”
4. The culprit is significant
The murderer must be a character we’ve spent time with, not a servant mentioned once in chapter two.
This is why the “butler did it” became a cliché and then a joke-it feels like a cop-out.
The Locked Room: The Impossible Crime
The locked room mystery takes fair-play to its extreme: the crime appears physically impossible.
The classic setup:
- A person is murdered
- The room is locked from the inside
- No other exits exist
- No one could have entered or left
- Yet someone did
John Dickson Carr: The Master
Carr (also wrote as Carter Dickson) was the king of locked room mysteries. His novel The Three Coffins (1935) includes a chapter titled “The Locked Room Lecture” where Dr. Fell discusses every possible solution to a locked room mystery.
Carr’s categories of solutions:
- Not actually locked: It appears locked, but there’s a trick to the lock mechanism
- Murder before the room was locked: The victim was already dying/dead
- Suicide disguised as murder: The victim staged it
- Accident disguised as murder: No crime occurred
- Hidden entrance: Secret door, panel, passage (must be established early)
- Mechanism from outside: String, ice, pulleys, etc. to manipulate the lock
- The killer never left: They’re still in the room (hidden or disguised)
Modern additions:
- Virtual locked rooms (digital surveillance showing impossibility)
- Time-delayed mechanisms
- Accomplice who locks the door after
- The “locked room” is an illusion created by witness testimony
Fair-Play Locked Room Design
If you’re writing a locked room mystery:
1. Establish the constraints clearly
Make sure the reader understands what makes the crime impossible. If the solution hinges on a window being large enough to climb through, describe the window’s size early.
2. Play fair with the mechanism
If your solution involves a icicle dagger that melts, you must mention:
- The cold temperature
- The presence of ice
- The wetness found at the scene
Don’t withhold these clues and spring the solution as a surprise. Fair-play means the reader could theoretically solve it.
3. The solution should be clever, not absurd
Rube Goldberg contraptions that require seventeen things to go perfectly right strain credibility.
The best solutions are simple in retrospect-“Oh, of course! Why didn’t I see that?”
The Structure of Fair-Play Mysteries
Act 1: The Setup
- Establish the victim, suspects, and setting
- Show the relationships and tensions
- Plant early clues (disguised among red herrings)
Act 2: The Crime and Investigation
- The impossible crime occurs
- Detective arrives and examines evidence
- Witnesses are interviewed
- More clues are revealed
- False solutions are considered and eliminated
Act 3: The Solution
- The detective gathers everyone
- Eliminates the impossible
- Reveals the logical deduction
- Proves the culprit’s guilt
- Explains how all the clues fit together
The Critical Middle
The middle of a mystery is where fair-play lives or dies.
You must:
- Present all crucial clues
- Create red herrings that are plausible but wrong
- Show the detective’s reasoning process
- Let the reader try to solve it alongside the detective
The balance:
- Too many clues = too easy
- Too few clues = reader feels cheated
- Too much misdirection = frustration
- Too little misdirection = boring
The Art of the Red Herring
A red herring is a false clue designed to mislead. But in fair-play fiction, even red herrings must be fair.
Good red herring: Points to an innocent person in a way that’s plausible but ultimately explained.
Example: A character has motive, opportunity, and no alibi. They’re clearly hiding something. But the thing they’re hiding (an affair, embezzlement, etc.) isn’t the murder.
Bad red herring: A clue that seems important but is never explained or is explained away as irrelevant.
Example: The detective finds a mysterious symbol carved at the scene. It’s never mentioned again. (Readers will feel cheated-why waste their attention?)
The Rule of Red Herrings
Every red herring must ultimately be explained, even if the explanation is “that was a coincidence.”
Nothing can feel forgotten or ignored.
Modern Fair-Play: Expanding the Rules
Contemporary mystery writers honor the spirit of fair-play while expanding what’s possible:
Unreliable Narrators (with caveats)
Gillian Flynn’s Gone Girl: Multiple POVs, one of whom is deliberately lying. This works because:
- The reader knows something is off
- Clues to the unreliability are present
- The revelation recontextualizes rather than contradicts
The rule: You can have an unreliable narrator, but the reader must be able to detect the unreliability from the clues.
Psychological Complexity
Modern mysteries explore why as much as how.
Tana French’s Dublin Murder Squad: The mysteries are fair-play, but the detectives’ psychological damage affects their investigation in visible ways.
Genre Blending
NK Jemisin’s The City We Became: Fantasy elements, but the mystery of what’s happening follows fair-play principles-clues are present, solutions are deducible.
When to Break the Rules
Rule-breaking that works:
Agatha Christie’s The Murder of Roger Ackroyd (1926) Breaks one of the sacred rules in a way that shocked readers but is now considered a classic twist. (No spoilers here.)
Why it worked:
- Christie planted subtle clues
- The solution was technically fair
- The rule-breaking itself was the innovation
Rule-breaking that doesn’t work:
Most “it was all in their head” endings Revealing that events didn’t happen as described feels like a cheat unless this possibility was clearly established.
Solutions requiring specialized knowledge If the solution hinges on knowing obscure botanical facts, and the reader had no way to learn them, they’ll feel cheated.
The Reader’s Experience
A great fair-play mystery creates a specific experience:
Phase 1: Confusion “How is this possible? This makes no sense.”
Phase 2: Investigation “What are the clues? What do they mean? Who had motive and opportunity?”
Phase 3: Attempted Solution “I think I’ve got it! It must be X!”
Phase 4: Doubt “Wait, that doesn’t explain Y. Maybe it’s Z?”
Phase 5: Revelation “Oh! It’s so obvious now! The clues were there all along!”
Phase 6: Retrospective Appreciation “Let me re-read and see how I missed it.”
If your mystery delivers this arc, you’ve succeeded.
Practical Exercise: Design Your Locked Room
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Choose your impossible scenario:
- Locked room, impossible disappearance, perfect alibi, etc.
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Create a logical solution:
- How was it actually done?
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Establish the constraints early:
- What makes this seem impossible?
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Plant the clues:
- What evidence points to the solution?
- Can a careful reader deduce it?
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Add red herrings:
- What false solutions are plausible?
- How will you eliminate them?
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Test it:
- Show it to a beta reader
- Ask: “Do you feel cheated, or do you feel you could have solved it?”
The Promise
Writing fair-play mystery is one of the hardest disciplines in fiction because you’re making a promise:
I will not cheat you. The solution is here. You can find it if you’re clever enough.
This requires:
- Meticulous plotting
- Careful clue placement
- Respect for the reader’s intelligence
- Restraint in withholding information
But when done right, the reader’s satisfaction is unmatched.
They didn’t just read a story-they solved a puzzle.
And that feeling-the “aha!” moment when the impossible becomes obvious-is why locked room mysteries have endured for a century and will endure for centuries more.
Play fair. The reader will thank you.
Next in the series: Horror’s Three Fears - Stephen King’s hierarchy of gross-out, horror, and terror, and how to deploy each for maximum impact.