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."
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. Alternatively, it can be a way for a Minor Crime to reveal a Major Plot, if investigating the Big Secret turns out to be more interesting and consequential than solving the original minor infraction.
- In the anime Love Hina, the episode "The Case of the Missing Hinata Apartment Money: A Mystery", 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.
- Yoshihisa has one during the second half of Episode 7 of Kotoura-san. This is very much unlike him since he normally doesn't keep secrets at all. It's actually a Surprise Party for Haruka's birthday which is celebrated at the end of the episode. The fact that he managed to keep this away from Haruka for those two weeks was extremely badass on his part!
- 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 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, she was getting liposuction at the time of the murder, 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. She eventually manages to out the real culprit via The Perry Mason Method, by pointing out a contradiction in her alibi, (And her hair).
- 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 illiteracy. 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.
- 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 keeping with the above, Vimes muses in Night Watch that part of the basis of police work is questioning people that privately believe cops can peer into their souls to find the Big Secret. You're just not allowed to slam their fingers in drawers until they give it up.
- Several examples from Dorothy L. Sayers' Lord Peter Wimsey novels:
- In 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, making her eventual decision to testify a serious danger, and also giving the Duke a motive to kill the dead man. Fortunately, Lord Peter manages to locate the dead man's suicide note, and the jealous husband's attempt to kill her results in his accidental death.
- In Have His Carcase, a boat was off shore when Harriet found the body. The owner was surly and obstructive, and his grandson had gone off to Ireland. When they had constructed several theories involving this boat, the grandson reappeared and explained that he and his grandfather had been poaching on another fisherman's lobster pots.
- In Whose Body?, the man in whose apartment the body was found eventually reveals his alibi: he had been persuaded to go to a nightclub.
- In John Dickson Carr's Death Watch, the police are investigating a murder in a house, and announce that they are going to make a thorough search of everyone's rooms. A very prim elderly spinster — who was completely above suspicion until then — suddenly starts acting extremely agitated, almost throwing a hysterical fit. Naturally, this draws attention to her. It turns out this very proper lady was hiding a cache of pornography magazines under her bed.
- In Charles Dickens' 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... which is that he is a doctor and not a soldier, much to his shame. The other characters are European and most of them don't understand that this might be why he was hiding that, making him seem so much more suspicious. 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 "The Boscombe Valley Mystery" the main suspect refused to tell police what he had argued with his father about moments before he died: He was married and didn't want the woman he loved (who was in the courtroom) to know.
- In "The Man With the Twisted Lip", a beggar is caught as a suspect in murder. He keeps silent until Holmes deduces his Big Secret:the beggar is the supposed murder victim - apparently, it pays much better than regular work.
- 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.
- In Gone Girl, Nick Dunne's suspicious behavior results in him becoming the prime suspect in his wife's disappearance. It turns out that Nick is having an affair with a younger woman.
- In Isaac Asimov's The Currents of Space, a man is accused of a crime. The man accusing has been Mind Raped by the criminal and only remembers that the latter used the accused one's name, and seeing the man towering over him. The suspect subverts the trope immediately by revealing his very well kept Big Secret - extremely short legs which make it physically impossible for him to tower over anyone.
- In One Of Us Is Lying, the murder victim Simon specialised in rooting these out and publishing them on his gossip app, About That, revelling especially in taking down the popular and powerful. The dirt he was about to publish on the Bayview Four, coupled with the fact that they were with him when he died, make them the main (only) suspects in his murder.
- Simon is caught at his own game when Jake Riordan finds out he fixed the Junior Prom vote so he'd end up on the court. The threat of being humiliated as he's done to so many others triggers the events of the story.
- The sequel concerns a game of Truth or Dare, in which the next "player" has twenty four hours to accept the Dare, or their Big Secret will be texted to the whole school. This is intended to force people to accept the Dare, thus allowing Brandon's murder to look like a dare gone wrong.
- In an episode of the fourth season of 24, James Hellar's suspected/interrogated son isn't in league with the terrorists after all... but he is guilty of attending an anti-war rally and of being gay.
- Boston Legal:
- One episode featured a case in which the secret was that the man was reluctant to use his alibi of having been in bed with another man.
- The same secret was used in another episode, 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.
- Season four of Dexter. Dexter didn't kill his wife, Rita, but he DID kill a whole lot of other people...
- Doctor Who: In "The Unicorn and the Wasp", as a homage to the works of Agatha Christie, there are many: Lady Eddison's Mysterious Past, Colonel Hugh's able-bodied state, Robina Redmond's real identity, and Roger's relationship with one of the footmen.
- 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.
- Monk, "Mr. Monk and the Other Woman". The reason Derrick suddenly went missing is that he was suffering from schizophrenia, and two years ago, he went to a private clinic in Zurich. Monk doesn't find this out until he asks Monica if she's guilty of the murder. They wanted to keep the news from his family.
- In the Murder, She Wrote episode "Simon Says, "Color Me Dead", Irene's reluctance to bring her discovery of the body to law enforcement suggests to Jessica that she's hiding something. The "something" turns out to be the fact that she isn't biologically or legally Tommy's mother. She had become close to his birth mother, Irene Rutledge, who died bringing him into the world. She told the nurses she was Tommy's aunt, allowing her to take him, and has been on the run using Irene's identity ever since.
- 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? Turns out he was sleeping with the girlfriend of one of the other peace protestors.
- Our Miss Brooks:
- In "Stolen Aerial", Miss Brooks is able to get a discount from a wolfish television repairman who wants to go out with her. Miss Brooks is advised to keep her discount a secret so as not to get many more freeloaders wanting the same deal. Too late, Mrs. Davis and Walter Denton had already let the cat out of the bag. Miss Brooks gets deluged with broken aerials and even television sets needing repair. However, Miss Brooks keeps her promise; she hides the real reason for her sudden television-equipment windfall from Mr. Conklin. Unfortunately, the television repairman had accidentally lent Miss Brooks Mr. Conklin's checkered television aerial. Mr. Conklin concludes Miss Brooks is a "female Fagin", using Mr. Boynton and several high school students to run a television-equipment theft ring.
- "The Jewel Robbery": Mr. Conklin accidentally breaks a jewelry store window when carrying a bad a laundry to the cleaners. Conklin believes the police want him for the inadvertent vandalism. In reality, they're looking for a thief who had robbed the jewelry store a short time before.
- The fifth episode of Police Squad! has a recently released bomb-maker lying about his whereabouts on the night when someone planted a bomb that killed the judge who sentenced him to jail the next morning... because he had violated his parole by crossing state lines to go to a baseball game.
- 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 the client was the lawyer himself, who is married.
- 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. His Big Secret turns out to be that he lied about his heritage to avoid discrimination in Star Fleet: his heritage isn't Vulcan, it's Romulan.
- Star Trek: Deep Space Nine: In the episode "Dax," Curzon Dax is accused of treason and murder (and the Dax symbiont's current host, Jadzia Dax, is subjected to an extradition hearing over it). Although Curzon was innocent, Dax doesn't defend herself because at the time of the murder, Curzon was in bed with the murdered man's wife. The widow turns up to confess to the adultery to save Jadzia.
- 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.
- 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 often 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 and a lot of suspicion about their real secret: All the time he had his deformed son locked in the basement, who was the real culprit in his mother's death.
- Virtually every episode of Fillmore! has these come out at some point, usually just before the clue pointing to the actual culprit turns up. These ranged from a Book Dumb character turning out to actually be an A-student, to the suspected mascot-napper turning out to have pilfered a rival school's goat mascot.