: How could a criminal forget a crime? I'm toying with a scenario however it's crucial to the plot that the murder doesn't know they're guilty and has to work with their friends to try and

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“If once a man indulges himself in murder, very soon he comes to think little of robbing; and from robbing he comes next to drinking and Sabbath-breaking, and from that to incivility and procrastination. Once begun upon this downward path, you never know where you are to stop. Many a man has dated his ruin from some murder or other that perhaps he thought little of at the time.”
― Thomas de Quincey

There was a book--the title and author of which I have utterly forgotten--where a ferry captain is involved in smuggling; he is bribed to contrive to bring a van aboard the ship without its going through Customs. Halfway through the voyage (from the Netherlands to the UK, I think), he becomes convinced that the police at the destination have been tipped off. Panicked, he stages a bomb-threat and has his crew push the van off the ship into the North Sea.

Later, he learns that the van's cargo was illegal aliens.

Would something like that do? The person is convinced to commit a crime, but one that he does not realize will lead to a death (a burglary of vital medication or medical equipment perhaps?) At some point, he discovers the connection, which he cannot expose without condemning himself as an accomplice and co-conspirator.

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Total Recall They simply have a machine that does "memory implants" and "memory wipes." Based on Philip K. Dick's short story "We Can Remember It for You Wholesale."

Slight spoiler, mostly from Wikipedia:

Douglas Quaid is a construction worker who discovers that he is actually
a secret agent formerly named Carl Hauser. He travels to Mars to
uncover his true identity and why his memory was erased. Adventures ensue.

Worse spoiler, from me:

In the end, you are left wondering which person really exists. Is the
spy Hauser only a figment of Quaid's imagination, and everything that
seemed to have happened on Mars only the result of a malfunctioning
"vacation machine," exactly like his wife and friends were telling him
at the beginning? Did Quaid reject his mundane real-life for a much
more interesting dream-life? And if so, is he really worse off? Or,
was part of the Mars adventure real, but the spy Hauser was only a
memory implanted by the bad guy into Quaid, to use Quaid to ferret out
the resistance, as the bad guy mockingly tells Quaid 3/4 into the
movie? Or, is Quaid real, and Hauser was an implanted persona to
neutralize Quaid without killing him?

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Sleepwalking, as many have mentioned, is one way to pull this off. I also don't see why a night of heavy drinking is out of the question. So they woke up with a heavy hangover and their shoes missing (or something) one morning--so what? Especially if the character has a history of heavy drinking, this won't seem out of the ordinary.

But there's another option: what if they do remember everything about the crime, but don't realize its significance until later? In other words, the murder was a complete accident. Perhaps the victim died in a fire, and your character only realizes much later that the fire was started by a cigarette they forgot to put out. Like any detective story, the chain of events here can be as simple or as complex as you like, and it could give you plenty of opportunities to throw out hints and red herrings along the way. Plus, it requires no supernatural elements, drugs, amnesia, sleepwalking, split personalities, or other unusual elements--just simple carelessness or ignorance.

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Protein synthesis is required for memory consolidation, so you can use protein synthesis inhibitor to block formation of long term memories in theory. See Gold. Protein synthesis inhibition and memory: formation vs amnesia (2008) for example.

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Let your character be a greedy, busy bounty hunter. He does everything as long as it is well paid. Murder, surveillance, investigation. All for money. He does not really cares about who he kills and who he helps.

The guy deserves to end up inquiring into one of his own old murdering contracts!

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Going at a slight tangent, Derren Brown once convinces a man that he committed a murder (which he did not), and forgot about it.

The episode is called "The Guilt Trip" from the series "The Experiments".

He does this through conditioning and triggers to invoke a feeling of guilt. Everyone around him are actors and he uses them to start messing with his memory and make him doubt his own mind. Getting actors to change clothes when he's not looking, swapping food plates...etc

He sets up a motive and then uses a drunken night and the triggers to convince him of the murder.

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I have seen a similar scenario on the TV show House MD which goes like the following:

A person sets up a situation where they bring the target into a situation that seems safe, however their unconscious adds in an element that they "want", and turns out to be dangerous for the target, that in hindsight the person knew it was dangerous, but not at the time they made the decision - it simply wasn't on their mind.

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You say that making the character drunk wouldn't work because there'd be evidence, e.g. a hangover. But so what? If someone woke up with a hangover, he'd know he'd been drunk, maybe he'd realize that he didn't remember anything he did the night before, but it would be quite a leap from "I was so drunk I don't remember what I did last night" to "I must have murdered someone".

Ditto various other drugs.

The idea of a brilliant detective being a drunkard or a drug addict seems a little out of character to me, but you could always come up with a reason why this one time he did something out of character, like an extreme personal problem. Or someone deliberately drugged him. (Ooh, suddenly gave me an amusing idea: A woman wakes up to realize that someone has given her a date-rape drug. While she's trying to figure out just who did it and what he did to her, she finds out that while she was drugged, she murdered someone. Like, she's thinking she was the victim and then it turns out she was the criminal.)

People who have amnesia don't remember what they did. That's pretty much the definition of amnesia. You'd have to give some explanation for it -- you can't just say, "oh, by the way, he had amnesia".

If it's a science fiction story, you can always invent some machine that erases memories. Then you can introduce the machine early in the story and have it be a part of the plot way before you reveal that it was used on the detective.

To follow up on someone else's suggestion: Is it necessary to your story for the hero to have deliberately killed the person? If not, then it could be some sort of accident and he doesn't realize anyone was killed. Ranging from "I wonder what this switch does?" to "I thought I hit someone with my car in the dark but there's no sign of a body", like because the person was thrown somewhere that the hero can't see the body, or maybe he runs away and dies somewhere else.

BTW saying this or that scenario would leave evidence: That would be a good thing, so there is some foreshadowing. You don't want to get to the final scene in the story and then with absolutely no warning say, "And then he noticed that the glove found at the scene was a perfect match for the one glove in his own coat pocket. He suddenly realized that he himself was the murderer. The end." There have to be some clues building up to this point. An early scene where the detective has a period of time that he doesn't remember what he did, which is then explained away in some plausible manner, and then brought back at the end, if done properly could be a nice piece of foreshadowing.

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2 ideas:

There was a planed murder (poison/trap/bomb) and the Character interupted the plan... and unknowingly triggered the Dead condition .. prepared by somebody else.

taking poisoned drink to some one else..
detonated a bomb with a phone call..
dropped a banana .. and 2 minutes after, someone behind him broke his neck...
Just work out a nice chain of events.

2.This is bit cooler: Commiting a crime by NOT acting.
- a friend calls "It is urgent please tell me...", "sorry dude no time for this now I'm at work". Two days later you get the news your friend is dead ...
- "Son please go to see the old lady next door, she asked for some help".

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I might suggest the idea of fugue states. While more commonly caused by drug or alcohol abuse, they can also be caused by epileptic seizures.

My father experienced Grand mal seizures and if he had a seizure while no one else was around, was sometimes found later with no memory of the seizure or the time before or after it. Later in life, when he was on a number of heavy medications to address them, he specifically began doing (or attempting to) dangerous things directly after a seizure (like standing in front of an oncoming train believing that he was impervious). He had no conscious memories of any of his activities or even his seizure.

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Some bacterial/viral diseases have been known to affect the formation of new memories (see the famous case study of Clive Wearing).

Perhaps a character is yet unaware of their diagnosis, and things in their life start "slipping through the cracks".

Lack of attention promptly after an event can also interfere with the formation of memory. Perhaps something startling happens to the character exactly after or even during the fatal event. This is a little far-fetched unless the killing itself is somewhat accidental or even incidental, as such a powerful emotion (associated with committing a major crime) would dominate one's attention. But for example, someone could "bump" something on the road, be concerned about it, but before checking up on the matter receive a phone call from one's mum. Two weeks later the character hears about a hit and run and finally realizes it was that fateful bump. Quite a moral dilemma here whether or not to turn oneself in. Perhaps the character calls back her mum to see if she remembers anything notable from the time of that call--looking for clues/salvation/deniability. What a taught conversation that would be. hth.

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Plot of Angel Heart, he was effectively reborn when he sold his soul

or

He commits the murder by accident, e.g. he wonders what a switch does, flicks it nothing happens. That causes someone to be electrocuted outside (but with a few layers of indirection)
Time travel, he hasn't killed him yet
Something dangerous laying around that without thinking he tells a child to just get rid of, that child posts it through the neighbours letter box, e.g. peanut butter, allergic reaction follows
Similar where he tells someone to 'just take care of it', later he finds that the person took care of it by cutting power to a building, the building hosts life support (e.g. a criminal will leave a building so cutting power stops the lifts, he's lazy so won't go out)

etc

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Is your story based in the present day?

You mentioned Poirot which is of course based during WWII.

If your story based in the past as well, you could maybe work medicine into this, specifically medical practices which used to be valid and have since been discredited as harmful.

Maybe your character thinks they are helping the victim by providing a certain medical treatment, the victim dies, the coroner asserts that they have been poisoned, and then the main character sets out to track down the perpetrator, only to find out it was them?

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Off the top of my head, solmnambulation and memory-impairing drugs are probably the easiest and most probable. Human memory is somewhat frail. In real life this generally impacts the accuracy and availability of memories, but yes, under the right circumstances people can fail to remember having done something. Many things affect memory, and with a little research I'm certain you could find at least ten scenarios that I'm not even thinking of right now.

I am working off of the assumption that you're not interested in trauma-related memory suppression.

1) Sleep-walking is not uncommon, and people who sleep-walk are able to perform all sorts of complex actions while immersed in slow-wave sleep. Look up homicidal sleepwalking on Wikipedia, it's a hoot. As a bonus some medications can cause sleep-walking as a side-effect, including a certain popular insomnia aid.

2) Certain drugs can cause memory loss or poor memory formation. One such is GHB which has gained some notoriety for its use in date rape. It's also occassionally found to have been used in cases of mugging and the like, and it can be difficult to detect. There are certainly other drugs that affect memory, too.

3) Just in case, there is always the possibility of post-traumatic memory loss. In fiction (especially movies) one commonly finds this trope in conjunction with hypnosis. Memory repression hearkens back to Freudian psychoanalysis. This choice has a strong whiff of pop-psychology, and hypnosis therapy in particular is highly controversial.

Hopefully these options can give you something to chew on.

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I believe this is part of the plot of the movie (spoiler below)

Memento

although I'm going on the Wikipedia summary, as I've never seen it.

I've read at least one sci-fi short story (dystopian future) where criminals went to a black market memory wiper and had their memories erased. The criminal became a different person; that "person" isn't the one who committed the crime.

So I suppose the post-wiped person could become a detective who specializes in cold cases, and she eventually comes across a murder which is starting to sound naggingly familiar but she can't put her finger on why...

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