Lights Off, Somebody Dies

The characters are attending a Masquerade Ball, a dinner party, or a similar event with a sizeable amount of people. Suddenly, all the lights go out, usually followed by chaos as the people scream and try to feel their way around in the dark.

The lights come back on, and someone has been murdered during all the commotion. The characters now have a mystery on their hands - in the dark, anyone could have done it.

In Real Life, this kind of murder would be very difficult to actually pull off. The murderer would have to be able to see in the dark, in order to avoid tripping over things or bumping into people. Then they would have to correctly identify the person they want to kill without seeing their face. (This sometimes leads to a plot twist where it turns out that somebody else in the room was actually the murderer's intended victim.) They'd also need to work out how to discreetly get the lights off in the first place - everyone is going to suspect that shady-looking fellow who was hanging around the light switch. Best not to think about it too hard.

Compare Darkness Equals Death, where the lights are already off.

Anime & Manga

 * Quite a few murders in Detective Conan rely on this trope, including one subversion where it turns out  kills the victim. ( !)
 * In Tantei Gakuen Q, there's a murder while a Phony Psychic has the lights turned off as part of a ritual.

Film

 * Mr. Boddy's murder in Clue. Mr. Boddy's actually the one who turns off the lights, expecting everyone in the room to attempt to murder someone else. It Makes Sense in Context.
 * In Devil, nearly every time the lights go out, someone gets murdered.
 * In Devil, nearly every time the lights go out, someone gets murdered.

Literature

 * In the Miss Marple novel A Murder Is Announced, a personal ad appears in the town newspaper, announcing that a murder will take place in a certain house at a certain time, and inviting everyone to come over. The Genre Blind locals assume the ad is inviting them to a party game (sort of "Murder in the Dark" for adults) and attend. Naturally, the owner of the house claims to know nothing about the ad, and then at the right time the lights go off...
 * Something of a subversion considering the person who is found dead is the person who was waving the gun around.
 * In Baynard Kendrick's novel "The Last Express" a killer stabs a man to death in a night club and no one notices because

Live Action TV
"Morla: The lights were out. Anyone would've had time to kill the lady."
 * Doctor Who has the Weeping Angels, lifeforms that actually function by this trope. They exist as statues when being observed, and only when no one is looking can they move. When the lights go out, they can move almost instantly from place to place.
 * This also happened in the episode "The Unicorn and the Wasp."
 * Star Trek: The Original Series episode "Wolf in the Fold". While a seance is going on to find out who the murderer is, the flame that's the only light in the room goes out. The medium running the seance screams, and when the light comes up again, she's dead: stabbed in the back.


 * One episode of The X-Files had an enemy that would only attack in the dark.
 * The Foyle's War episode "The White Feather"
 * An episode of Family Matters has an imaginary parody of detective works with Urkel as the detective. Every time Urkel would accuse someone of the murders, thunder would make the lights go off briefly and that person would be found dead.
 * Parodied in Monty Python's Flying Circus. In one sketch the lights go off and a police inspector is killed. Another police inspector enters and decides to reconstruct the crime by having the lights turned off. While they're off, he is killed as well. The scenario repeats until there's a large pile of dead police officers in the room.
 * Used in the slasher-themed episode of Boy Meets World.
 * Used in Tracker. One of the fugitive aliens cut the lights in the Watchfire, then killed someone while the lights were off.

Music
"And then suddenly there, all the lights went out, and a voice cried "Die you must!"
 * The song Dangerous Dan McGrew. Barbershop quartets sometimes sing a variation, part of which goes:

And a shot rang out and a woman screamed. Somebody bit the dust!

Then the lights flashed on and the Northwest Mounted Police came a crashing through

And they drew their guns, and they said "Which one is Dangerous Dan McGrew?""

Western Animation
"Leela: Could you get the lights on your way out?"
 * Invoked In-Universe by Bart Simpson. After telling his class mates a ghost story, he reassures them that Dark Stanley would never dare attack a crowded, well-lit -- The lights go out, then back on, and Bart is "dead" on the ground with exposed brains.
 * In the first "Tales of Interest" episode of Futurama, Leela's last three victims are all killed this way.


 * The South Park episode "Cartman's Mom Is Still A Dirty Slut" begins with Dr. Mephesto being shot in the chest. It becomes the focus of the entire episode.
 * In the Codename: Kids Next Door episode "C.L.U.E.S.", Numbuh Three's Posh Party Rainbow Monkey is stabbed in the back this way.
 * In the episode "Mystery Train" of Adventure Time, it's exaggerated and subverted. A lot of candy people die on the train one by one as the lights go off, leaving nothing but skeletons. Finn tries to solve the mystery, while it turns out that

Other

 * The children's game "Murder in the Dark" is based on this trope.