Locked Room Mystery

A seemingly impossible crime. The standard example being that of a murder victim found in a room with only a single door, securely locked from the inside. Can be the basis for a single plot, or an entire show. A well-designed Locked Room Mystery provides pleasure from trying to figure out the puzzle before it is revealed, from moments of dawning realisation, and from a satisfyingly logical solution. A poorly designed Locked Room Mystery only provides a feeling of having been cheated. Contrary to the name, Locked Room Mysteries don't necessarily have to be locked rooms (e.g. contemplating how it's possible for someone to travel from one part of the island to another within minutes).

Originally from crime fiction, John Dickson Carr being an acknowledged master. It is noteworthy that Edgar Allan Poe's short story The Murders in the Rue Morgue, widely considered to be the first detective story, involves a Locked Room Mystery.

Appears on television in a number of forms. The relatively pure form as a sub-genre of crime television (e.g., Monk, Jonathan Creek) where the puzzle is eventually unravelled by an eccentric protagonist using subtle clues and pure reason.

The part of the show where the solution to the mystery is explained is The Summation. One common trick used is the Time-Delayed Death. Expect the police to have found that The Key Is Behind the Lock when the crime scene was discovered.

Anime and Manga

 * Although it is not a crime show, the anime Spiral has a number of locked room mysteries that the protagonist must solve, including one literal locked room murder.
 * The Suzumiya Haruhi two part episode "Remote Island Syndrome" (and the corresponding chapter in one of the light novels) has a fairly brilliantly-executed example of this, inside a locked room on an island hit by a terrible storm.
 * Detective Conan frequently uses these, though it also plays with this trope, as Conan solves a case when he realizes that a man died, but the rest of the room was set up to make it look like a locked room mystery.
 * Similarly, The Kindaichi Case Files features one almost every story.
 * Also commonplace in Tantei Gakuen Q.
 * One of these came up in Kuroshitsuji.
 * Golgo 13, "The Serizawa Family Murders". Two witnesses (brother and sister) to an old murder agree to a meeting in a hotel room guarded—but not under surveillance—by the police. They go in, but when the police storm the room after several hours, only the brother is in the room, denying that he ever saw his sister that evening. Despite being the prime suspect, they have to release him because they never find even a trace of the sister's body.
 * Lampshaded in Domu: A Child's Dream, where the detectives have to investigate a series of suicides at one apartment complex. One man jumps off the roof to his death, despite the lock of the door to get onto the roof being rusted shut for years on end. The younger policeman points out how ridiculous this "sort of... locked roof mystery, right?" is.
 * One of these pops up in Franken Fran to a group of former patients of Fran, with Okita specifically calling it by the trope name. The solution is a lot squickier than some of the other answers.
 * In the Ace Attorney manga, Turnabout Showtime is referred to as "the world's smallest locked-room murder", when Flip Chambers is somehow fatally stabbed inside his Sparklestar costume, which cannot hold anything in its hands and is impossible to open by the wearer (although it comes to light that there are workarounds for that, such as catching the zipper on a piece if wire, or wearing the costume backwards).
 * Inverted on Bokurano. . Of course, nobody except the kids and the higher-ups on the military know that. Then again,.

Comic Books

 * Donald Duck and especially Mickey Mouse comics occasionally feature some versions. Unfortunately, it's poorly executed much of the time; if it seems the crime could only have been committed by a thief who could turn invisible, it's likely that's exactly what they did somehow, using a gadget or magic spell. However, there do exist some decent stories that feature an actual mystery of this sort as well.
 * A staple of The Maze Agency comic book series.
 * The central case in Bookhunter involves three concentric locked room mysteries: The thief entered a locked library, removed a book from a locked safe, and carried the book out past the alarm checkpoints—leaving so little evidence that the theft wasn't noticed until weeks later.
 * There's one featured in Batman #700, where the significantly aged corpse of Carter Nichols is found dead in his basement. Batman and Robin are unable to solve the case, but it is later shown that

Film

 * To some extent, I Robot fits this trope: Dr. Alfred J. Lanning's death looks like an open-and-shut suicide because the door to his room was locked. Spooner, of course, thinks otherwise. This is
 * Lampooned in the movie Murder By Death, which pretty much makes fun of all fiction crime characters - Sidney Wang modeled after Charlie Chan, Sam Diamond modeled after Sam Spade - and their methods of baffling their readers.
 * Inverted in Law Abiding Citizen, in that Clyde is somehow pulling off elaborate murders while locked in solitary confinement.
 * Teddy Daniels is brought to Shutter Island to investigate a locked room mystery - in this case someone escaping from a locked and guarded room.

Literature

 * The original locked room mystery is The Murders in the Rue Morgue, by Edgar Allan Poe. The story became the Trope Codifier for later detective murder mysteries.
 * The deuterocanonical Old Testament story Bel and the Dragon has similarities to a locked room mystery.
 * The most famous Sherlock Holmes locked room mystery is probably "The Adventure Of The Speckled Band".
 * Arguably, "The Adventure of the Empty House" was more popular.
 * Before that, there was The Sign of Four
 * Discworld:
 * Parodied in the short story "Theatre of Cruelty", in which Vimes's Internal Monologue brings up the complaint that "wizards made locked room mysteries commonplace" (though there are never any actual examples of this happening in the series).
 * The ultimately non-fatal poisoning of Lord Vetinari in Feet of Clay is a completely fair locked room mystery. It has an interesting partial subversion in that
 * Happens in the Finnegan Zwake series with the man in Finn and Stoppard's storage room (in Horizontal Man) and Professor Freaze in his tent (in Worm Tunnel).
 * Intentionally played to the point of absurdity in The Long Dark Tea-Time of the Soul by Douglas Adams. Dirk Gently finds that his newest client, a wealthy man who had hired him as a security guard, had his head severed and placed on an active record machine while awaiting Dirk. Of course, the door to the room was locked from the inside when the scene was initially discovered. The police analyze this as an elaborate suicide done simply to cause trouble.
 * Occurs in Jeffrey Deaver's novel The Vanished Man, where the killer is seemingly able to escape from a locked room where one of his victims is found, as well as disappear into a small crowd.
 * Played with in Jasper Fforde's The Big Over Easy, a detective story using nursery rhyme characters. "The entire crime-fighting fraternity yesterday bade a tearful farewell to the last 'locked room' mystery at a large banquet held in its honor. The much-loved conceptual chestnut of mystery fiction for over a century had been unwell for many years and was finally discovered dead at 3:15 A.M. last Tuesday."
 * Then it turns out that the locked room mystery was murdered... in a locked room.
 * In her non-fiction book Sex Crimes, Alice Vachss (wife of crime writer Andrew Vachss) mentions a real life case that she prosecuted, where the accused was locked in his room every night by nuns, and so supposedly could not have committed the rape. In the end the jury decided that any youth in that position would have found a way out of his room long ago.
 * John Dickson Carr, the acknowledged master of this back in the golden age of crime fiction, provided all sorts of different ways to accomplish this. In his book The Hollow Man/The Three Coffins, the main character actually gives a lecture on the different ways a locked room mystery can be created. If the detective is Gideon Fell, Henry Merrivale, or Henri Benicolin, there is an excellent chance you've got a locked room or impossible crime on your hands.
 * Randall Garrett used this trope often in his Lord Darcy stories, with the added twist that magic is real in Darcy's world. Magicians naturally become prime suspects in a Locked Room Mystery, yet Lord Darcy often works out a non-magical explanation, thus exonerating some innocent wizard of the crime.
 * Death in the Dawntime, by F. Gwynplaine Macintyre was written specially for The Mammoth Book Of Historical Detectives, and probably has the earliest setting ever for a detective story. It's a Sealed Cave mystery.
 * The Mammoth Book of Historical Whodunnits opened with the Ancient Egypt-set The Locked Tomb Mystery by Elizabeth Peters, which is Exactly What It Says on the Tin.
 * Stieg Larsson's Men Who Hate Women/The Girl With the Dragon Tattoo, is about a journalist investigating a forty year old murder which is a Locked Room Mystery on an island.
 * It's a subversion, though. The girl in question did live on an island, and she did disappear without a trace during a time period when the only bridge to the mainland was closed off. However, her uncle later admits that the family didn't realise that her disappearance could have been against her will until the bridge was already open again, and by then she and the abductor could have been long gone.
 * Ellery Queen had a number of locked room mysteries, including The King is Dead.
 * The eighth Ellery Queen mystery, The Chinese Orange Mystery is a locked room mystery with exceedingly weird clues, including the fact that the murder victim is found with his clothes on backwards.
 * The Psych tie-in novel "Mind over Magic" centers on the case of a costumed magician who appears to dissolve into nothingness in a water tank before a crowd of spectators but never reappears after the illusion, with a dead body suddenly in the tank in the magician's place.
 * Amelia Bones's body was found in this fashion by the Muggle police in Harry Potter, the police of course couldn't figure what happened, but the reader knew that she was a wizard killed by the Death Eaters.
 * One of the "girls" disappears from a third floor room in The Alienist.
 * C. Daly King's Obelists Fly High is a variant: the murder takes place on an airplane.
 * The novel The Mystery Of The Yellow Room, by Gaston Leroux (better known as the author of The Phantom of the Opera) is a Locked Room Mystery that is also a perfect Fair Play Whodunnit. The twist solution to the mystery, while being completely unexpected to the unprepared reader, manages to be on reflection the only logically possible solution given the facts of the case.
 * One of the characters in the prologue to Roger Levy's Reckless Sleep is a fan of locked door mysteries, and has thought up a fairly elaborate one of his own which he gleefully shows off to an uninterested accomplice.
 * The protagonist of the short story "The Man Who Read John Dickson Carr" by William Brittain murdered his wealthy uncle and escaped from the room through the chimney in order to confuse the inevitable investigation. It might have worked if he'd remembered to lock the door.
 * The murder in Gilbert Adair's Agatha Christie pastiche The Act Of Roger Murgatroid (whose title itself is a pun on Christie's The Murder of Roger Ackroyd) is of the locked room variety.
 * In the first installment of the Max Liebermann Papers by Frank Tallis, A Death In Vienna, the murder is committed and discovered in an actual Locked Room scenario.
 * One Tarma and Kethry short story featured a locked room mystery, with all of the locals assuming that the victim's wife was the killer, because she was the only person other than the dead man who had a key to the room in question.
 * Many of the Simon Ark mysteries involve some kind of variation on the locked room mystery.

Live-Action TV

 * A more diluted form sometimes appears in a Police Procedural (e.g., CSI) where the puzzle is eventually unraveled by an eccentric protagonist using more obvious clues and Applied Phlebotinum.
 * In genre television (e.g., The X-Files), the puzzle is typically subverted when an eccentric protagonist (Agent Mulder), on the basis of flimsy evidence and wild speculation, reveals that a Monster of the Week did it. In this case the Monster of the Week is the one using Applied Phlebotinum in the form of special monster powers.
 * The two episodes in Season 1, "Squeeze" and "Tooms", epitomize this trope variant, with the MOTW managing to get as his victims in otherwise inaccessible places through his mutant ability to squeeze through tiny vents, windows, chimneys, etc.
 * Jonathan Creek is a show built around various locked room mysteries and other seemingly impossible crimes. A few examples:
 * In "Danse Macabre", a murderer carries a hostage into a sealed room. When the door is opened, only the hostage remains. The solution:
 * In "The Scented Room", a painting vanishes from a locked room during a school visit. The solution:
 * In "Jack in the Box", a man is found dead in his nuclear bomb shelter, locked from the inside. Suicide... except he was arthritic and couldn't hold a gun. The solution:
 * In "The House of Monkeys", a scientist is found stabbed by a suit of samurai armour in his locked office. The solution:
 * In "The Omega Man", the army seize what a prominent Ufologist claims is an alien's skeleton. While locked in a crate to take it back to base however, it vanishes. The solution:
 * In "Ghosts Forge", Jonathan's assistant vanishes from a locked room while investigating a mystery. The solution:
 * In "The Three Gamblers", the corpse of a dead criminal seems to have climbed the stairs from the cellar where the body was left and scratched at the door. The solution:
 * Monk doesn't usually have crimes that look totally impossible, just ones that look impossible for Monk's prime suspect to have committed. The prime suspect is always guilty anyway. Some episodes turned this Up to Eleven, with one killer who was in outer space at the time of the murder, and still turned out to be guilty, and another where a guy managed to commit murder while in a coma. In one case, a criminal even tried to avoid blame for ordering a murder by provoking a witness report that seemed to indicate he had done it personally, which actually was impossible. There was also a more traditional locked-room crime in an episode where the victim was a magazine publisher who died in an apparent accident while working out in a locked room. . In another episode, a record producer is found shot inside a locked panic room which the police had to literally cut a hole in the door to enter, the only other living thing in the room being a Chimpanzee who was holding the gun when the police came in.
 * One Crossing Jordan episode involved a man who was writing a book on vampires found dead in a locked room, drained of blood and with fang wounds in his neck.
 * On Kamen Rider Agito, the hallmark of a killing by the Unknown is that it is, by all rights, impossible - drowning in the middle of a field, being buried deep underground without any signs of digging, being entombed inside of a tree, your internal organs being ripped apart without the skin being broken, or falling to your death in a place where there's nowhere to fall from.
 * The Star Trek: Deep Space Nine episode "A Man Alone" is a locked room mystery, with the added twist that the only DNA found in the room is that of the victim.
 * The mysteries on Banacek were all 'impossible crime' thefts; including locked room mysteries.
 * Most of the first few seasons of Murder, She Wrote, being heavily based on Agatha Christie and Little Old Lady Investigates, were full of these.

Tabletop Games

 * In Warhammer Fantasy, Deathmaster Snitch killed an Imperial noble that had locked himself into a tower with the only door being guarded by a large band of knights. The knights didn't notice anything until the following morning. In fact, no explanation is ever given for how he did it, making it even more of a mystery.
 * Easy. He climbed the walls of the tower.

Video Games

 * The second Ace Attorney has a case where your assistant is locked in an empty room with the victim, a gunshot is heard, and the door is opened to reveal her standing over the corpse with a smoking gun in her hand. Naturally, it now falls on you to prove her innocent in court.
 * Another case involves a man found dead in the middle of a snow-covered courtyard with only one set of footprints leading to the body... and an eyewitness who saw the killer leave the murder scene by flying over the rooftops.
 * And in Ace Attorney Investigations we get  that comes this way.
 * By the way, John Dickson Carr (see above) invented (or at least popularized) this style of "impossible crime" too.
 * In Apollo Justice: Ace Attorney there's a case where the killer has to have escaped from the room in the few seconds after Apollo and Ema heard the gunshots.
 * There are other cases such as "Bridge to the Turnabout" that seem pretty straightforward to begin with, until Phoenix finds something that points to the crime scene being somewhere else, leading to another day of investigation for both parties.
 * In one chapter of Ghost Trick, Lynne's corpse is found alone, in a room with only one way out that leads to several police officers waiting on the next floor. Of course there's no real mystery to solve, Sissel just has to go four minutes back in time to find out what happened.
 * An interesting inversion of this trope occurs in The Elder Scrolls IV: Oblivion during a side mission; You and five other people are locked into a mansion on the premise that hidden somewhere in the house is a chest full of gold. Whoever finds it gets to keep it. A fun little game between friends, right? But soon, people start turning up dead, one by one, and suspicions fly as to who the killer is. Now, here's the twist;
 * Oh, yeah, and
 * Unfortunately, it's programmed so badly that
 * Invoked Trope repeatedly in Umineko no Naku Koro ni, where it forms the core of the argument that the culprit must be the Golden Witch Beatrice instead of a human. Beatrice is knowledgeable about the classics and goes to some lengths to rule out the obvious solutions.
 * Bonus points to the first twilight of the third arc; six people are found murdered in six different locked rooms, and each one has the key to one of the other rooms on their corpse.
 * Trauma Team's Forensics mode features a stage with this trope's exact name.
 * Dr. Schrader's death in Professor Layton and the Diabolical Box is one of these. Layton concludes that.

Web Original

 * Reach's story (The Big Idea) in the Whateley Universe features one of these prominently. There's quite a bit of discussion about the ways in which one needs to examine such a mystery, before the final denouement happens more or less as expected.
 * Crime Fiction Resource site with locked room bibliographies.

Real Life

 * If criminal investigators encounter a Locked Room Mystery in real life, the usual default assumption (barring physical evidence to the contrary) is that the victim locked themselves in the room before succumbing to a Time-Delayed Death. Given how rare it is for an Instant Death Bullet and/or Poison to occur outside of fiction, and how a threatened person is likely to put a barrier between themselves and an attacker if they can, this sort of unintended Locked Room Mystery happens too often in the real world to really count as a "mystery".