Big Secret

"''"The Judge said 'Son, what's your alibi? If you were somewhere else, then you won't have to die.' I spoke not a word, 'though it meant my life. For I'd been in the arms of my best friend's wife."''"

- Lefty Frizzell, The Long Black Veil

A suspect in an investigation has a Dark Secret, completely unrelated to the crime at hand.

Because he is so obviously lying, or because there is such strong circumstantial evidence against him, the police waste a lot of time investigating him before he is cleared.

Sometimes he will finally give in and reveal his secret, but in other cases the police dig it up, or another character will speak up to clear him, even though it may implicate the supporting character. If a character acts reluctant but coughs it up fairly quickly, odds are good that it's Infraction Distraction.

A particular pattern involving adultery (e.g. the suspect has an alibi he can't use—he was in bed with a married woman and doesn't want to ruin her marriage) is very common and is currently being regularly subverted.

On TV, having a big secret usually means that you're not the murderer, although you may be guilty of other things.

This is a standard Red Herring.

See also: You Didn't Ask, Mysterious Past, Be as Unhelpful as Possible.

Anime and Manga

 * In the anime Love Hina, the episode "The Case of the Missing Hinata Apartment Money: A Mystery", Narusegawa Naru refuses to let the other residents search her room for the missing money and later grabs the turtle, Tama-chan, when it is seen to have a piece of paper stuck to it (which is thought to possibly be a piece of one of the bills). Of course, it turns out that Naru was not the culprit; rather, she did not want the others to learn that she treasured a photo of herself and Keitaro, which she had been looking for and which was the paper stuck on Tama's shell.
 * Detective Conan offers many many examples, and contains some subversions and aversions as well.

Film

 * In Legally Blonde, Brooke Taylor Windham (a famous fitness instructor) is accused of murdering her wealthy husband. She actually does have a concrete and easily proven alibi,, but the nature of said alibi would ruin her. Elle does manage to get her to reveal this, but due to a Delta Nu Oath, is sworn to secrecy. Thus she has to prove her clients innocence while withholding this piece of information.
 * In 'Catch a Fire', Patrick Chamusso is held and tortured by the authorities. He is suspected of a bombing at a power plant, but cannot acquit himself because he spent the time with the mother of his secret 'illegitimate' son.
 * In The Reader, Michael realizes that Hanna has made a series of strange, hasty moves, including working as a guard at Auschwitz, to conceal . Her continued efforts to do this result in her taking a far greater share of blame for an atrocity she was involved in than she should have, and those most responsible getting punished lightly.

Literature

 * Literary example: Almost always happens in Agatha Christie novels. Hercule Poirot sometimes comments that everyone has something to hide, and will therefore lie in an investigation.
 * In Dorothy L. Sayers's Lord Peter Wimsey novel Clouds of Witness, the Duke was committing adultery when his sister's fiance committed suicide. The other woman was married to a vindictively jealous man,.
 * In Have His Carcass, a boat was off shore when Harriet found the body. The owner was surly and obstructive, and his grandson had gone off to Ireland.
 * In Whose Body?, the man in whose apartment the body was found eventually reveals his alibi: he had bee persuaded to go to a nightclub.
 * In Charles Dickens's A Tale of Two Cities, Charles Darnay refused to reveal even at his trial for treason that he is searching for the child of a family his father wronged.
 * Gintaro Aono in Murder on the Leviathan has a Big Secret... . Likewise with some of the other suspects.
 * In Elizabeth Gaskell's North and South, Margaret is seen at a train station at night with a man. She cannot explain that he was her brother, as he is wanted for treason and was not supposed to be in the country.
 * In the Sherlock Holmes story, a Boscombe Valley Mystery,.
 * In Harry Potter and The Chamber of Secrets, Percy acts suspiciously because he doesn't want anyone to find out about his girlfriend Penelope Clearwater.

Live Action TV

 * An episode of Boston Legal featured a case in which the Big Secret was that the man was reluctant to use his alibi of having been in bed with another man.
 * The same Big Secret was used in another episode of Boston Legal but made way more believable by the twist that it was one of Denny's first cases and had happened during the 1960s. Denny, being awesome, managed to get the man acquitted without revealing the truth.
 * In an episode of the fourth Season of 24 James Hellar's suspected/interrogated son isn't in league with the terrorists afterall... but he is 'guilty' of attending an anti-war rally and of being a Gay.
 * The Star Trek: Deep Space Nine episode "Dax" is a typical example, albeit with an atypical defendant.
 * Star Trek: The Next Generation: "The Drumhead" features an example. After sabotage leads to an investigation of everyone on board the Enterprise, the investigator's chief suspect becomes Ensign Tarses. A young man of mixed human/Vulcan heritage, it becomes clear he's hiding something and the investigative team is convinced he's behind the sabotage. Unfortunately for everyone, especially Ensign Tarses, it turns out that he's completely innocent of involvement in the original crime.
 * In an episode of The Practice, a friend of Bobby's (who is also a lawyer) is protecting a prostitute in a murder case. The prostitute has an alibi (she was with a client at the time), but the client doesn't want to be revealed. The truth is that
 * In any given episode of New Tricks, expect three or four of these to come tumbling out before the murderer of the week is rounded up.
 * One particularly beautiful example: one episode revolves around the 1984 murder of a peace protestor outside a NATO military base during an anti-nuclear protest. This, naturally, led to a lot of Big Secrets being dug up about spies and military 'spooks', government moles planted within the peace organization to inform on and discredit them, and secret plans to sabotage the military base's communications system—and all of it revolves around a 'secret' that the victim apparently had. After all of this, what, then, was the Big Secret that got the victim killed?
 * Every episode of House: Member of family gets sick. Cause unknown. History taken, however investigation reveals: a) she was taking drugs, and didn't tell anyone b) she was sleeping around c) and got pregnant d) AND an STD e) AND she's a dude. The clinic scenes, often a microcosm of the whole show, are this to the extreme: patient walks in with a sniffle, walks out with divorce papers.
 * Seems to be where this season of Dexter is going. Dexter didn't kill, but he DID kill a whole lot of other people...

Music

 * The song "Long Black Veil" is about a man who is hanged for murder because he refuses to reveal his alibi—he was having an affair with his best friend's wife.
 * The Gary Moore song "Over the Hills and Far Away" (covered more recently by Finnish band Nightwish) describes a similar situation—the main character is sent to prison for a robbery he didn't commit, because he'd spent the night in the arms of his best friend's wife and refuses to reveal the secret. Interestingly the reason he was convicted was because his pistol had been found at the scene, implying that someone knew about it and framed him knowing he wouldn't say it wasn't him.
 * Hypothesis: The husband did it, to break up the lovers without having to confront or kill his best friend.

Video Games

 * Commonly used in the Ace Attorney games, though Phoenix and Apollo usually manage to prove some point with the secret once it has been revealed.
 * A oft used trick is for the defendant to be an accomplice to the murder, either blackmailed or hoping you can get them off without them having to go to jail themselves.
 * In one particularly brilliant subversion, Phoenix manages to get a suspect to reveal their Big Secret, to the surprise of the court...and then the Judge pretty much says So What, and Phoenix immediately flounders when he realizes that the Big Secret was entirely worthless.
 * Scratches. On the backstory the former owner of the mansion (where the game is set) was accused of murdering his wife since an eyewitness told that she saw him carrying her corpse to the yard, since he died soon after and the mansion passed to his best friend who was also a respected doctor the police was left with no evidence but a lot of suspicion about their real secret.