The investigators put the evidence together, find out who did it, and then the perp gets off scot-free because the evidence isn't enough to convict them in court. This typically involves something being excluded from the trial, or seeing the crime happen on screen so that the audience is in no doubt about the suspect's guilt.
How this works out depends on where the story is set. In a American-based story, if the main protagonists are law enforcement officials and the like, you can usually expect them to find some new piece of evidence or get a confession that allows them to proceed with the conviction. If the story is more focused on the culprits or is bleaker in outlook, the verdict will effectively be found innocent, leading to a clear-cut case of Karma Houdini.
This trope takes on different meaning in a large number of non-U.S. countries (including Scotland, whose courtroom laws serve as the Trope Namer), where the principle of double jeopardy isn't observed. This means that the criminal could ostensibly be brought to trial again at a later point, if new evidence was found. Thus, it isn't quite as dramatic in works set in those regions of the world.
Compare Conviction by Contradiction, where the detectives appear to erroneously believe the evidence is enough for a conviction. See also Proof Dare, when the perp taunts the detectives for their lack of evidence.
- In Death Note, Kira directs Raye Penber's attention to an ordinary-looking man mopping the floor at a Starbucks-like coffee shop, explaining that this man has faced rape charges multiple times, but each time, he walked because there wasn't enough evidence to convict him. Kira then kills the man via his usual Hollywood Heart Attack method right in front of Raye's eyes, to show him that he is Kira, and he won't hesitate to do likewise to Raye if he doesn't cooperate.
- In Gundam Build Divers, Kyoya Kujo confronts an old friend of his, the admin Game Master, over the Mass-Diver problem infesting the Gunpla Battle Nexus Online game. Despite these people blatantly cheating, ravaging the digital world they live in and being caught on video doing so, Game Master reveals that, for some strange reason, their data logs do not record this data. To the game, they are playing fairly. This makes Kyoya's friend Rommel feel that the maker of the Break Decal programming the Mass-Divers use to cheat is modifying the code within the main database.
- Sirius Black became an example of this trope in "My Parents' Secret Keeper". He got acquitted but the wizarding world believed him to be guilty. Dumbledore was so afraid Sirius would go after Harry he even asked Remus Lupin to spy on him. Fortunately, while Dumbledore and Arthur Weasley (who was the Minister of Magic in that fic) were trying to persuade Lupin into spying, Percy appeared to tell Arthur about Scabbers being missing (Peter wouldn't stick around with an acquitted Sirius around) and the way Percy described Scabbers to Remus made the werewolf know the truth.
- This was not the only fic where Sirius becomes an example of the trope. In "These Grim Bones", because Fudge was afraid Sirius might have revealed some of the muggles caught at the explosion could have been saved had the obliviators noticed them, all Sirius could do was answer questions regarding his own guilt. Namely, he said he didn't kill Pettigrew, he didn't kill those muggles and didn't betray the Potters to Voldemort. Despite the fact Sirius stated it while under Veritaserum, Dumbledore still believed Sirius to be guilty and, being unable to prevent Sirius' acquittal, persuaded Fudge into sending a Hit Wizard to (officially) protect Sirius from people who might decide to take justice on their own hands but (extra-officially) find and report evidence to send Sirius back to Azkaban. Fortunately, the witch who took the job was Amelia Bones, who did it so the job wouldn't be taken by someone who, hoping for a quick promotion, would forge evidence. She believes Sirius to be guilty but she's too honest to approve convicting anyone (even a Death Eater) on false evidence.
- In "Growing Up Black", when some relatives of Sirius Black found out he was never allowed to plead his case in a trial, his maternal Grandfather argued at the Wizengamot that Dumbledore's testimony was just hearsay and testimony from muggles was inadmissible. Then, when asked if he wanted Sirius to be given a trial, his Grandfather instead invoked a law stating that, as a pureblood, Sirius shouldn't be forced to stay in Azkaban for more than 30 days without a trial and, because he was, the charges he'd been sent there for should be dropped. People who still believe Sirius to be guilty usually state he got Off on a Technicality.
- In "Three Dursleys, a Potter and a Black", Sirius was acquitted and Dumbledore mentioned the blood protection so Harry wouldn't be taken away from the Dursleys. Taking advantage of the fact that Dumbledore never ordered him not to move into Privet Drive and that the Dursleys love money more than they hate magic, Sirius moved into their home. When Dumbledore found out Sirius reached Harry and the Dursleys despite the wards keeping them from being found by anyone meaning to harm Harry, he assumed Sirius used some dark magic to get past the wards.
- The All Guardsmen Party is cleared of criminal incompetence in allowing their two Astartes handlers to be crushed by a Tyranid flyer and bisected by a Hive Tyrant, respectively by a Ordo Xenos tribunal. Two-thirds of it, anyway.
- The film Loving Vincent leans on recent theorizing that Vincent van Gogh's death was not a suicide when Armand Roulin is tasked with delivering one of Vincent's letters to Theo van Gogh and ends up in Auvers (the place Vincent died). The doctor who examined van Gogh talks a great deal about angles and distance, and Armand finds circumstantial evidence that the shooting might have happened somewhere other than the wheatfield; meanwhile Vincent's therapist Dr. Gachet insists it was suicide. Though Armand concludes that it was an accidental shooting which Vincent lied about, there's not enough evidence (and he's not a policeman anyway), and Gachet's view is sincere because he blames himself.
- In the Fade: André and Edda Möller's trial ends in their acquittals, because the judges find there's enough reasonable doubt that it's required. The chief judge however is explicit that they do not believe them innocent.
- The Lord Peter Wimsey novel Have His Carcase ends with Peter and Harriet knowing who committed the murder, how it was done, and that it would be stunningly difficult to prove it. Harriet, however, points out that the reason for the murder was about to arise again, so they do decide to go to court over it..
- In Harry Potter and the Goblet of Fire, it was revealed that, when Voldemort killed his Muggle relatives, the residents of the village where they lived were sure the victims' gardener, Frank Bryce, was the murderer, despite the police having to release Bryce due to being unable to prove the victims were murdered in the first place (the Killing Curse doesn't leave marks that can be detected by non-magical forensics). He lived under suspicion for the rest of his life, and ultimately died when Voldemort returned to the village to hide out one night and he subsequently went to investigate.
- In The Disappeared, this leads to a multicultural kerfluffle. Under the Rev legal system, a lawyer is essentially obligated to be a character witness for the accused, and if the accused is found innocent and convicted of a later crime, the lawyer is an accessory. One human lawyer made a huge legal sensation by defending a client when the court couldn't prove his guilt on the specific charges, but when the crook was later found guilty of something else, the court system turned against the lawyer.
- In Kris Longknife: Furious, Kris is tried for crimes against humanity on Musashi.Explanation (spoilers for Daring) The panel of judges returns a verdict of "not proven", which her attorney explains as being along the lines of, they're not convinced she did the right thing but they don't think the prosecutor made a convincing argument either.
- In Honor Harrington: Field of Dishonor, due to Jury and Witness Tampering by Earl North Hollow, the panel of admirals adjudicating his son Captain Pavel Young's Court Martial (for insubordination and cowardice) are split fifty-fifty. As a compromise, Admiral Hemphill proposes that they declare themselves unable to reach a verdict on the most serious charges (which carry the death penalty), meaning Lord Young is dishonorably discharged but still alive to seek revenge on Honor.
- CSI quite a bit.
- An episode of CSI: Miami rests on avoiding this. The CSI have evidence linking a person to a drive-by, but the key evidence is a testimony from the victim's son, who is a child and whom no one wants to put in front of a jury. The CSI race to find more evidence to make up for the kid. They fail and he walks free from killing the kid's mom... Though he is immediately arrested him for the murder of his accomplice, and this time it's an airtight case.
- Law & Order: Special Victims Unit does this a bit too. Sometimes it's that the perp gets off from something the main characters think should be illegal, but actually isn't (see There Should Be a Law for examples), but in other cases, someone who did (or likely did) commit a crime still walks. In most cases, something ultimately happens to prevent the perpetrator from going on their merry way (either they're re-arrested for another crime, or they end up on the wrong side of a Vigilante Execution), but there have been a few genuine Karma Houdinis as well.
- The biggest Karma Houdini of them all is probably Darius Parker, who walks on a double murder he confessed to by getting his confession and the subsequent evidence tossed and then dirtying up SVU to tarnish the credibility of everything else.
- William Lewis walks on several crimes before he's finally caught enough in the act for the charges to stick. Even then, there was a moment where he looked like he'd get away with that one too.
- In the Law & Order: UK episode "Samaritan" (based on the Law & Order episode "Manhood"), the team discover a homophobic policeman waited for four minutes to assist his gay partner who was caught in a crossfire and died- the gunman was also killed). The CPS decide that they're never going to make manslaughter by gross negligence stick and instead go for misconduct in a public office. Unfortunately, the CCTV footage of him hanging around is too grainy to identify him and is thrown out by the judge for being prejudicial because of it, the medical experts can't say whether arriving four minutes earlier would have saved him and the officer who knew he did it refuses to testify. The copper is acquitted.
- NCIS had one case where the perp gave instructions to the gang from their absent leader. The team knew he had killed a sailor and the supposedly absent leader, but couldn't prove it. So they showed the other gang members their evidence, mentioned that they would never get a conviction, and the perp shows up the next morning dead in a dumpster.
- Cold Case had one episode end with this, in which a prominent politician admits to Valens, off the record, that he committed the murder years ago. Unfortunately, his sister, in a misguided show of loyalty ( and possibly one-sided love), has already confessed to everything, and there's no evidence to contradict her claims.
- Justified plays this straight and subverts it when the kidnapping and attempted murder charges against Boyd Crowder are dropped. The testimony of the only witnesses, Ava Crowder and Raylan Givens, is deemed tainted because they started sleeping together and can be used to easily create reasonable doubt in a jury. Boyd did intimidate Ava into staying in the house till Raylan arrived so the kidnapping charge is valid (though difficult to prove), however, Boyd drew his gun in response to Ava pointing a shotgun at him so the attempted murder is primarily (it is implied that he was still deciding whether to try and kill them) a case of self defense.
- The reason the prosecutors rely on the kidnapping and attempted murder charges so much is because they have no evidence to convict Boyd of the charges he is 100% guilty of: bank robbery, blowing up a church and murder.
- Boyd's father Bowden is convicted of the charges he is guilty of but the sheriff who arrested him is found to be working for a Columbian drug cartel so a retrial is ordered. The prosecutor decides not to bother proving the case again since Bowden only has a few months of his sentence left anyway.
- In season two, murder charges against Dicky Bennett are dropped after the only witness recants his testimony. The case would still have been difficult to prove since the witness was an accomplice and the physical evidence is more incriminating toward him than Dicky. Everyone knows what actually happened but an unbiased jury would have probably acquitted barring more incriminating evidence.
- An episode of Criminal Minds has the team arrest a man who fits the profile perfectly, and who has no alibis for any of the murders, but as there isn't any definitive evidence to say he did it, the team aren't sure what to do with him. After he is released, the team enact a plan to get him to confess, but it goes horribly wrong and results in his death and the death of someone else, and they still don't have any proof that he did it.
- Baltar's trial in the rebooted Battlestar Galactica ends with a not-guilty verdict that Adama (who was the swing vote on the judges' panel) explains this way, saying to Roslin that "not guilty is not the same as innocent" and that the verdict is because "the defense made their case; the prosecution didn't". A good decision, considering that Baltar happened to be innocent of the specific charges brought against him.
- In Blindspot, Kurt's father spent the last 25 years with half the town, Kurt included, convinced he had abducted and murdered Kurt's childhood friend Taylor Shaw, though the police couldn't prove anything. He's absolved early in season 1 by the revelation that Jane Doe is Taylor Shaw (proven by a DNA test) and based on the timeline had to have been abducted by someone else entirely. While the season 1 finale shows this test to have been faked, he fesses up to Kurt mid-season that he didn't alibi out because he was ashamed of himself for having attempted suicide—which happened at the same time that Shaw went missing.
- Monty Python's Flying Circus: A court case has the Honourable Mr Justice Kilbraken on trial:
Clerk: If I may charge you, m'lud, you are charged m'lud that on the fourteenth day of June 1970 you did commit acts likely to cause of breach of the peace. How plead you, m'lud, guilty or not guilty?
Judge Kilbraken: Not guilty. Case not proven. Court adjourned. (He hits the dock; everyone proceeds to walk off until the main judge stops them)
- In the second Laura Bow game, even if the player has figured it out already, the murderer will get off scot-free if you haven't collected enough evidence to prove it. In the first game, the murderer ends up dead regardless of what you do, collect, or figure out.
- Most Ace Attorney cases seem like they'll end like this, until the protagonist puts everything together at the last second. There are a few exceptions:
- The Terry Fawles case from the third game. Rather than testify against Dahlia, the woman who framed him and who he still desperately loves, Fawles committed suicide.
- Apollo Justice: Ace Attorney:
- In the third case, Apollo knows who to accuse but his case hits a wall when he lacks anything concrete enough to get the guilty party to admit to their guilt. The only way he can slam the book on the actual culprit is by having the defendant admit to his own culpability in a different but related crime. The other crime carries a harsh sentence in his homeland but not in Apollo's country, which makes it advantageous to him to confess now instead of being found guilty at a later time. In doing so, the murderer would be revealed as the two were co-conspirators on the latter crime. After threatening to blow the lid on the whole affair, the real guilty party loses it and breaks down.
- The final case revolves around this: the murderer has covered his tracks well enough to leave no direct evidence linking himself to the crime, but all the other facts at hand point quite definitively at him. The problem is, the existing court system in place in the series' universe requires either evidence, a confession, or incredibly strong testimony...which is why Phoenix has spent the past seven years working towards the re-establishment of a jury system.
- Happened in Ace Attorney Investigations with Manny Coachen. He was never convicted of murdering Cece Yew because The Syndicate that he worked for stole the evidence from the prosecution.
- One case of Dai Gyakuten Saiban involves this, but in a twist it's your client who gets off on this... right as Ryunosuke (Phoenix's ancestor) starts thinking he may be guilty after all. Unfortunately, he's done such a good job defending his client that the judge and jury feel there isn't enough evicence to be certain of the defendant's guilt, and the trial ends in a Not Guilty verdict. Turns out he is guilty after all... but he's promptly murdered right after he walks free.
- When Pepper first met Tony in Iron Man: Armored Adventures, she told them that, after the plane crash where Tony's father disappeared, the FBI spent a whole year investigating Obadiah Stane as a suspect and dropped the case since they found no proof of either guilt or innocence. Tony spent the first season believing Stane was guilty until Gene Khan privately admitted to him he was the real culprit.