Taxonomy of locked room murdersWed 24 May 2023
In fiction, a locked room murder is a murder where the body is discovered in a place from which it would seem impossible for the murderer to have escaped undetected. For example, in a room that is locked from the inside, or a room that was watched by a security guard the entire time, or a room where the only exits were covered by CCTV. (We could imagine an accompanying taxonomy of locked rooms). There are really only a few ways such a murder could have been carried out. This post contains spoilers for some Jonathan Creek episodes.
The enjoyment from a locked room murder is in trying to work out how the crime was committed, rather than who did it. Generally all of the clues are revealed as the story progresses, and we are encouraged to try to solve the problem on our own before the big reveal.
What follows is all the different ways I could imagine a locked room murder could be carried out. I present the tropes very roughly in order from most to least promising, in my estimation.
1. The room is booby-trapped
The killer (deliberately) left a rigged object, poisoned food or drink, or a deadly animal inside the room, or tampered with the room itself in some way, such that the victim would be killed some time after they locked the door. (If it's not deliberate, see type 8 instead.)
This is my favourite type of locked room murder, and I think it has the most scope for an interesting story.
Examples from Creek include:
- In "Mother Redcap", people die of fright when looking out of a particular window in an inn. It turns out that there are electrified nails in the floor and the victims are being electrocuted.
- In "The Grinning Man", the bath tub tips its occupants into a tank of water once it has enough weight in it.
- In "The Coonskin Cap", the stab vest is rigged to inflate to suffocate the wearer, before deflating again in time to be discovered. (This is a bit different because the booby trap is on the person rather than in the room, see also type 5 since the vest is commanded to inflate by radio control).
2. The victim sustains an injury before locking themselves in the room, and succumbs later
This could be from a physical attack, or they could have been poisoned prior to entering the room.
I think the seminal example is the 1907 French serial Le Mystère de la Chambre Jaune (The Mystery of the Yellow Room), which features in the Creek episode "The Letters of Septimus Noone". In "The Letters of Septimus Noone", The Mystery of the Yellow Room is turned into a stage musical. In a parallel to the stage musical, one of the actresses is stabbed for real, locks herself in her dressing room, and subsequently dies in the locked room.
This is a clever idea, but there just aren't all that many ways to give someone an injury that they will later die from.
3. The victim is still alive when discovered, is killed by the discoverer
The discoverer kills the victim upon "discovering" them, and then declares it to be an impossible crime because the room was locked upon their arrival.
The discoverer should ideally appear implausible as a murderer, or have a method that makes it appear as though the murder took place substantially before the discovery, otherwise the possibility that they did it is quite obvious.
4. The murderer hides inside the room and sneaks out once the coast is clear
Something about the initial investigation of the room is not sufficiently thorough, leaving the murderer the opportunity to remain hidden and then escape later.
For this to be a viable strategy, the killer must know that the initial investigation of the room will not discover them, otherwise it's too risky. Maybe they know it will be someone who will be in shock at the discovery of the body, and will immediately leave the scene to seek help.
5. The murder was committed from outside
The victim locks themselves in a room and the murderer kills them without entering the room, and ideally makes it look like the murder could only have taken place from inside the room.
Maybe a gun is fired through a letterbox. Maybe poisonous gas is piped through an air vent.
6. The murder took place elsewhere and the body was dumped in the room
The room must have some method of entry that you can chuck a dead body through, but which a living person could not use. For example, maybe the room is deep underground and the only doorway has been bricked up for years, but for some reason there is a skylight or something directly above the room that the body could be dropped into.
Probably hard to make a good story out of this, because as soon as we learn about the entry method, it will become clear that a body could have been dumped through it.
7. The method of watching the door is defective or malicious
The security guard falls asleep. The security guard is the murderer. The "witnesses" were in on it. The CCTV is hacked so that it loops innocuous footage of nothing happening, instead of the real footage of the murderer coming and going.
If you're writing a locked room murder and you don't plan to use this trope, you would probably actually lock the room, rather than leave this possibility open, since the murder is so obviously trivial if the "watched room" is not watched properly. So if the room is merely watched, rather than locked, then it's either the case that the watching is defective/malicious, or the writer explicitly wants this to appear as a strong possibility.
8. It's not a murder, it's an accidental death
There is no murderer, the death just happened by accident after the victim had locked themselves in the room.
Maybe the victim banged their head on something, maybe they suffered a heart attack, maybe they didn't know that some innocuous-looking object in the room was actually deadly.
9. The room was actually locked from the outside
The murder took place conventionally, then the murderer left the room and locked the door in such a way that it appeared that the door had been locked from the inside.
10. The room has a secret exit which the murderer used
The murderer goes into the room through the normal entrance, commits the murder, locks the door, and escapes through a secret exit which nobody else knows about. Or maybe the victim was already in the locked room, and the murderer both entered and exited through the same secret passage.
Similarly to type 6, it's hard to make this into a good story, because either the reader knows about the secret passage or they don't. If they know about the secret passage then the solution is trivial, and if they don't know about the secret passage then they don't have a fair chance of solving it.
11. It's not a murder, it's a suicide
The victim locked themselves in the room and then killed themselves.
For this trope to be at all compelling, we probably either need to be looking to explain why the person committed suicide, or it needs to appear impossible as a suicide. Suicide is the default assumption when a dead body is found inside a locked room, so there has to be something else, otherwise we just have "It looked like a suicide, it was a suicide, the end".
In the Creek episode "The Seer of the Sands", a man kills himself (albeit not in a locked room) after receiving a fax containg good news. Why? He misread the fax because a tiny insect had landed on it which appeared to place a comma in the middle of a sentence, changing the meaning completely.
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