That One Case

""The case that makes you and the case that breaks you… The one you never solve, the one that keeps you awake at night. The case that gnaws at your guts and ruins your marriage. The case that keeps you propping up a bar as you relive the what-ifs, the might-have-beens, the half-leads and half-truths. The case that other cops murmur about whenever you walk past. The case you never… ever... discuss.""

- Herschel Biggs, LA Noire

A veteran detective is haunted by that case they could never solve (or perhaps it was solved officially, but the guy thinks something isn't right) and it haunts them to this day. Nearly always a serial killer (or at least a very gruesome murder) and/or personal. Even when they are officially off the clock, they will still work the case on their "free" time, because it is that important to them. If they are told to leave the case alone they may get fired over it, but still continue even when they are not a cop anymore. Sub Trope of My Greatest Failure.

Truth in Television: A number of homicide detectives have solved cold cases that were haunting them or are haunted by the still unsolved ones. Generally murdered children cases.

Anime and Manga

 * Case Closed has quite a few of these haunting the police department:
 * Kogoro's That One Case ended with a Shoot the Hostage; he quit and separated from his wife soon after this. The Non Serial Movie "The Fourteenth Target" circles around the suspect supposedly taking revenge for this incident.
 * Sato has two That One Cases, both of them "inherited": in one, her father perished chasing the suspect from a bank robbery, and in another, she lost her former partner to a serial Mad Bomber (who himself had an earlier That One Case that lost his prior partner).
 * Subverted with Megure's That One Case, where a serial hit-and-run driver critically injured both himself and the girl who offered herself up as bait--turns out that was how he met his wife and he's embarrassed to tell the tale.
 * Conan himself considers the death of  and his inability to stop the murderer of the Moonlight Sonata from committing suicide his greatest failures.
 * Runge in Monster believes that Dr. Tenma is a Serial Killer and chases after him relentlessly. It gets to the point where his wife and daughter leave him.

Comics

 * Sin City: That Yellow Bastard is all about That One Case for John Hartigan: a serial child rapist/murderer who happens to be the son of one of the most powerful people in Sin City.
 * An occasional form of plot in Batman comics.
 * In Gotham Central, a young detective is dragged into a cold case when the survivor of a horrible school bombing from years before specifically asks for him before killing himself. When he and his partner re-open the case, which had never been solved, they have to bring in Harvey Bullock, the cop who originally worked the case and who has most of the evidence in his personal possession. It is revealed that this is Bullock's One Case, which he claims he "brought home with him" even after he left the force. Bullock's inability to solve the case, even now, pushes him over the edge.
 * In the Nightwing series, Nightwing resolved the One Case of a cop in time for the man's retirement party, so he wouldn't let the unresolved case eat him alive.
 * During the Bruce Wayne: Fugitive? storyline, Batman was called to the bedside of an old dying cop who had been the one to talk to Bruce after he'd witnessed the murder of his parents. The man asked Batman to clear Bruce Wayne's name in memory of the case he'd never been able to resolve, saying that the child he'd seen on that night could not have grown to be a murderer.

Film

 * The Pledge is based on this premise, with the detective's obsession with his last case eventually driving him insane.

Literature

 * Paula Myo chases Adam Elvin and the Guardians of Selfhood in the Commonwealth Saga by Peter F Hamilton. The case is not so much unsolved as open, because she can't catch the perpetrators - but it is firmly established that it is Paula's only case not closed by an arrest and conviction. The child/gruesome murder angle applies, but not as you might think.
 * The Girl with the Dragon Tattoo features two; the murders of Rebekka Jakobson and Harriet Vanger. Both of the cases were fretted over by different cops, to the same effect; even long into retirement, neither case was solved nor dropped by the head investigator until the titular Salander came along.

Live Action Television

 * Cold Case revisits a suspicious death that happened during the Civil Rights era which haunts Jeffries because he grew up in that neighborhood and discovered the victim's body as a boy.
 * The nature of the show gives most of the detectives at least one of these, possibly more.
 * There was also at least one episode where the original investigator of the team's latest cold case was the one who brought it to the team's attention in the first place.
 * The episode "Unfinished Business" of Murder She Wrote involved a police officer just about to retire who announced he'd be reopening the investigation into the murder of a DA at a resort.
 * This led to  in CSI New York. She obtained her one case, got fired after considering forging evidence to put the serial rapist away and in a later episode.
 * In Monk Trudy's murder is the only one the main character has not solved, and it's haunted him for many years. He only manages to solve it in the Grand Finale.
 * Veronica Mars has this with the Lily Kane case in Season 1.
 * Law and Order Criminal Intent: Sort of an informed That One Case, Alex's husband was shot in the line of duty, and a later case made a plot point out of the circumstances surrounding said shooting.
 * The main character in The Mentalist with the "Red John" case, which resulted in the murder of his wife and child.
 * In the House episode "All In," House chases the medical case that he was never able to solve: a young boy comes in exhibiting the same symptoms as a patient House earlier lost.
 * Criminal Minds has a couple of these:
 * Agent Ryan, though not a regular, is still obsessed with a serial killer case he never solved.
 * Agent Rossi, who is a regular, has been haunted most of his career by the brutal murder of a couple in a house while their children were sleeping, then cleaned the murder weapon and any forensic evidence and left without waking them..
 * Hotch has The Boston Reaper.
 * Reid had a weird example: the murder of Riley Jenkins. It actually occurred when he was 4 years old and he'd consciously forgotten about it. However, he had a recurring nightmare about investigating the murder of a child, and it turned out to be Riley Jenkins in the dream. When he realized this, he consciously agonized over the case until he solved it.
 * Martin in Frasier continued to pore over the Weeping Lotus Case during retirement, and eventually solved it with a bit of help from Frasier. Admittedly Frasier's aid consisted of accidentally rearranging the crime scene photos so that Martin realized what he'd been missing, then coming up with a theory that the woman had been killed a by gorilla trained to use a gun (she wasn't).
 * In "Homicide", Bayliss had the Adena Watson case.
 * Truth in Television, unfortunately. The real Baltimore child-murder which inspired the Adena Watson storyline was never closed. Getting word out about That One Case is one of the reasons why the real-life Baltimore Homicide Division was supportive of the true-crime book about their work.
 * Inspector Morse has a subverted example: the short story "Morse's Greatest Mystery" is about the 'one case he never solved' which is something as minor as someone stealing Â£20 from a Christmas charity box. Morse claims to have got the culprit, but it turns out he just replaced it with his own money. It turns out the "mystery" of the title refers to the cynical, materialist Morse beginning to understand altruism.
 * Played with in Castle; Det. Beckett's mother was murdered, providing her Freudian Excuse, but she decided to stop looking at her mothers case because it was driving her crazy, and forbids anyone else from looking at it. Still unsolved as of this point.
 * As of S2E13,
 * The Fringe episode "Earthling" deals with Broyles's That One Case from years ago that he obsessed over so much it destroyed his marriage. The team eventually solves it, though it turns out to involve a government conspiracy and a Living Shadow.
 * The episode "Name" of Law and Order Special Victims Unit has Stabler team up with a random Lab Rat, Millie, to help solve her deceased father's One Case--the murder of an unidentified Hispanic boy. The case so deeply affected Millie's father that he'd memorized the boy's fingerprints, helped pay for his gravestone, and obsessed over both solving the murder and identifying the victim until he died. In the end,.
 * The fourth season of Dexter revolves around ex-Agent Lundy trying to track down the "Trinity Killer" who has murdered three people a year for the last fifteen years, who he was never able to convince his colleagues actually existed..
 * The NCIS episode Lt. Jane Doe has Ducky cancelling a trip to a conference in Britain to stay and investigate when a murder victim turns out to have a trident mark on her neck (and be a civilian dressed in naval uniform), just like the victim of a 10-year-old unsolved case he worked on.
 * Cary of Unforgettable is forever haunted by the one case she couldn't solve from her detective work from her home city - the murder of her older sister Rachel. What makes it harder on her is that she saw the killer, but due to Trauma Induced Amnesia, that crime scene is the one moment that a woman with otherwise perfect memory cannot remember.
 * Cary of Unforgettable is forever haunted by the one case she couldn't solve from her detective work from her home city - the murder of her older sister Rachel. What makes it harder on her is that she saw the killer, but due to Trauma Induced Amnesia, that crime scene is the one moment that a woman with otherwise perfect memory cannot remember.

Video Games

 * Ace Attorney has the SL-9 and DL-6 incidents, while Apollo Justice has.
 * There's also the KG-8 case in Ace Attorney Investigations. The twist is that it actually did get solved  but the case was dismissed due to lack of evidence.
 * The sequel, Gyakuten Kenji 2, introduces the IS-7 incident,
 * In Persona 4 the Hierophant social link (and to a lesser degree, the Justice social link, where it is mentioned in passing) reveal that Ryoutarou Dojima's that one case is.
 * In Fahrenheit, there is Sergeant Mitchell's Kirsten case. Although the murderer has been apprehended on site and convicted, Mitchell later discovered that exactly the same ritualistic murders have been going on for years and nobody batted an eyelash. Needless to say, the murder that Lukas committed and that Carla investigates follows the same pattern. Reeks of an Ancient Conspiracy?