->''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''', ''VideoGame/LANoire''

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 [[ItsPersonal 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 [[TurnInYourBadge they may get fired over it]], but still continue even when they are not a cop any more. SubTrope of MyGreatestFailure.

TruthInTelevision: 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.



[[folder:Anime and Manga]]
* ''Manga/DetectiveConan'' has quite a few of these haunting the police department:
** Kogoro's One Case ended with a ShootTheHostage; he quit the police and separated from his wife Eri soon after this. The NonSerialMovie "The Fourteenth Target" circles around the suspect supposedly taking revenge for this incident.
** Sato has ''two'' 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 [[spoiler: and TheLostLenore]] Matsuda to a serial MadBomber (who himself had an earlier That One Case that lost ''his'' prior partner Hagiwara).
** Subverted with Megure's That One Case, where a serial hit-and-run driver critically injured both himself and [[DecoyDamsel 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 since she is both a FormerTeenRebel ''and'' [[MayDecemberRomance much younger than Megure himself]].
** Conan himself considers the death of [[spoiler: Ai's older sister Akemi]] and his inability to stop the murderer of the Moonlight Sonata from committing suicide his [[MyGreatestFailure greatest failures]].
** Superintendent Matsumoto has his own That One Case. [[spoiler:During a chase following two murders, the biggest suspect of said killings rammed Matsumoto's partner and best friend Morimura with his car; Morimura bled to death [[DiedInYourArmsTonight in Matsumoto's arms]] while still trying to relay {{dying clue}}s. It's really not helped by the culprit being the one who gave Matsumoto his [[EyeScream infamous scar across his eyes]] few later]].
* [[ImplacableMan Runge]] in ''Anime/{{Monster}}'' believes that Dr. Tenma is a SerialKiller and chases after him relentlessly. It gets to the point where his wife and daughter leave him. [[spoiler:By the end of the series, however, he's given a second chance by his daughter.]]
* In ''Manga/BokuDakeGaInaiMachi'', TV reporter Sawada is convinced that the serial child abduction and murders that took place in his town eighteen years ago were not the work of MisunderstoodLonerWithAHeartOfGold [[MiscarriageOfJustice Jun Shiratori]] but of an experienced killer who had been active in cases prior to and since said murders. He still has a file on the case, which he later shows to [[ClearMyName Satoru]] to help him find his mother's (presumably the same) killer.

* ''ComicBook/SinCity: 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 Franchise/{{Batman}} comics.
** In ''ComicBook/GothamCentral'', 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 [[CowboyCop 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, [[DrivenToSuicide pushes him over the edge]].
** In the ''ComicBook/{{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 ''ComicBook/BruceWayneFugitive?'' storyline, [[SecretIdentityIdentity Batman]] was called to the bedside of an old dying cop who had been the one to talk to Bruce after he'd [[DeathByOriginStory witnessed the murder of his parents]]. The man asked Batman to [[ClearMyName 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.
** Similarly, depending on the continuity, the murder of his parents is Batman's One Case. It's referenced in ''WesternAnimation/BatmanTheAnimatedSeries'', ''WesternAnimation/TheBatman'', and ''WesternAnimation/YoungJustice'' that Batman wants to prevent the murder of Robin's parents from becoming that one case and ruining ''his'' life, too.

* ''Film/ThePledge'' is based on this premise, with the detective's obsession with his last case eventually driving him insane.
* Inspector Toschi in ''Film/{{Zodiac}}'' obsesses over his inability to catch the title murderer, to an extent that seriously jeopardizes his career. Definitely TruthInTelevision.

* Paula Myo chases Adam Elvin and the Guardians of Selfhood in the ''Literature/CommonwealthSaga'' by Creator/PeterFHamilton. 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.
* ''[[Literature/TheMillenniumTrilogy 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.

[[folder:Live-Action TV]]
* ''Series/ColdCase'' 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 ''Series/MurderSheWrote'' 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. [[spoiler:He was actually guilty and reopening the investigation as a way to kill a blackmailer and disguise it as an attempt on his life]].
* This led to [[spoiler:Aiden's death]] in ''Series/{{CSINY}}''. She obtained her one case, got fired after considering forging evidence to put the serial rapist away and in a later episode [[spoiler:is killed by said rapist while following him. She is able to leave a bite mark on him (which Mac would think to look for, as she solved a case this way in the past) and her former colleagues are able to put him away]].
** Mac also had a That One Case that involved the kidnapping of a boy; he finally solved it in "Exit Strategy".
* In ''Series/{{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 GrandFinale. Ironically, [[spoiler:he probably could have solved it any time he wanted to. All he had to do was open Trudy's last gift to him]].
** Stottlemeyer has one of his own in one episode. During the investigation of the world's oldest man's death, Stottlemeyer recalls the one case he couldn't solve involving a young man's hit and run death. [[spoiler:It turns out that the same man was responsible for both deaths]].
* ''Series/VeronicaMars'' has this with the Lily Kane case in Season 1.
* ''Series/LawAndOrderCriminalIntent'': 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.
* Patrick Jane, the main character in ''Series/TheMentalist'' deals with the "Red John" case. Unlike others, he caused the case to happen when he was pretending to be a psychic and insulted the serial killer Red John, which resulted in the murder of his wife and child. His life since has been the pursuit of finding Red John and killing him.
* In the ''Series/{{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.
* ''Series/CriminalMinds'' has a couple of these:
** Agent Ryan, though not a regular, is 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. [[spoiler:It turned out that the killer was a circus clown with a child's mind, who followed the children home to play with them and killed the parents because they frightened him after they found him in the house with a weapon the father was holding; the scene had been cleaned by the father of the clown to avoid sending his handicapped son to jail. They were caught because the clown had been sending childish presents like teddy bears every year to apologize]].
** Hotch has The Boston Reaper.
*** [[spoiler:Hotch ''had'' the Boston Reaper. He's closed That One Case now, albeit at great personal cost.]]
** 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 [[BadDreams 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 ''Series/{{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 by a gorilla trained to use a gun ([[CaptainObvious she wasn't]]).
* In ''Series/HomicideLifeOnTheStreet'', Bayliss had the Adena Watson case.
** TruthInTelevision, 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.
* ''Series/InspectorMorse'' 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 £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 ''Series/{{Castle}}''; Det. Beckett's mother was murdered, providing her FreudianExcuse, but she decided to stop looking at her mother's case because it was driving her crazy, and forbids anyone else from looking at it. Still unsolved as of this point. [[spoiler:Although Castle did make some headway, he lost Beckett's friendship because of it. StatusQuoIsGod, however, and getting back together is the emotional subplot of [=S2E1=]]].
** As of [=S2E13=], [[spoiler:Beckett found the killer, a professional assassin, but was forced to shoot him to save Castle...before he could tell her who had hired him. Castle tries to quit after this incident, but Beckett tells him that [[SugarWiki/HeartwarmingMoments she wants him around when they find the employer]]. Of course, if he tells anyone she said that there will be another shooting...]]
** In [=S6E9=], it's revealed that [[spoiler:the serial killer known as 3XK]] is this for Castle.
* The ''Series/{{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 LivingShadow [[spoiler:radiation-eating alien]].
* The episode "Name" of ''Series/LawAndOrderSpecialVictimsUnit'' has Stabler team up with a random LabRat, 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, [[spoiler:they link the case to several others and get the murderer, but he's already dying of cancer and it's been so long that he can't remember who the boy was. Millie blames her childhood resentment of the way the boy's case ate up her dad's time]].
* The fourth season of ''Series/{{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 ([[spoiler:later revealed to be four people a year for the past ''thirty'' years]]), who he was never able to convince his colleagues actually existed. [[spoiler:After he is killed for getting too close, Debra resolves to solve the case in his honor]].
* The ''Series/{{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. [[spoiler:It turns out that new murder was a crime of passion by the daughter of the original case's lead investigator, and she planted the evidence to link to the older case. The original killer (but not the original victim) is finally identified as a result of the new investigation]].
* 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 TraumaInducedAmnesia, that crime scene is the one moment that a woman with otherwise perfect memory cannot remember.
* ''Series/TheCloser'', has Philip Stroh, who Brenda becomes obsessed with. Stroh is a ''very'' AmoralAttorney who is also a serial rapist (and as it later turns out, a SerialKiller) who happens to specialize in defending sexual offenders, though this is clearly shown he does this so put all the blame of his crimes on them. Brenda is absolutely certain that he's guilty, but can't prove it and so she starts to follow him around and harass him, and even installs a GPS tracker in his car. Even to the audience, Stroh is incredibly creepy, mostly because as Brenda becomes increasingly desperate to get to him, he never loses his cool and actually seems to enjoy knowing that she's knows what he's done but can't do anything about it. [[spoiler:Right up until the end of the series, when he finally does something rash.]]
* ''Series/{{Bones}}'' has Pelant, who seems to keep getting away every time he shows up. [[spoiler:And when he went down, he left her with ''another'' case threatening to be unending]].
* In the premiere of ''Series/GoldenBoy'' we are introduced to the case that has been haunting Det Owen for the last eleven years. On the morning of September 11, 2001 a man was murdered and the police investigation quickly found some very promising leads. Then the planes flew into the Twin Towers and in the ensuing chaos the trail went cold. Owen regularly rereads the case file and keeps in touch with the victim's widow.
** Det Mackenzie is haunted by the unsolved murder of her brother, a police officer killed in the line of duty. While she has been able to partially move on, her retired cop father has been obsessed with the case and is desperate to find his son's killer. The case becomes hot again after the same gun is used in another shooting and this time the killer left evidence the cops can follow. [[spoiler:The killer was a mob enforcer and a serial rapist who shot the brother when he interrupted a rape attempt. The victim of the rape was so scared that she fled the city. When she returned a year later, the killer hired a mook to kill her and gave him the same gun. The mook screwed up and killed the witness's sister instead]].
* It applies to the ''Series/DoctorWho'' episode "Midnight", to an extent. We never see the true form or learn the nature of the MonsterOfTheWeek, and it's regarded as one of the eeriest episodes in the new series because of it.
* In ''Series/TrueDetective'', Cohle is convinced that the 1995 murder case he worked and apparently solved is still unfinished. He continues working it long past its closure, believing that there are additional perpetrators to be found and that the crimes are still ongoing. He's right.
* In ''Series/KamenRiderDouble'', it's the [[WhenItAllBegan "Begins Night"]] case where Shotaro and Philip met and became Double for the first time; but what makes it That One Case for Shotaro is that it was also when his boss and mentor was killed in action.
* C.D. Parker of ''Series/WalkerTexasRanger'' is met with the reopening of a cold case that retired him in the episode "Hall of Fame"--women in general. CD never caught the Hangman because he got checked into a clinic to try and mend his personality, but that failed and he went right back to killings. CD has a proper showdown this time.
* Harvey Bullock of ''Series/{{Gotham}}'' has the Goat murders. He caught the guy ten years prior to the start of the series, but his partner got put in a wheelchair in the process. When a killer with the same MO and signature crops up in the present day, the normally-lazy and cynical Bullock suddenly starts taking his job ''very'' seriously.
** In the same series, Jim Gordon, rookie detective, has the Wayne murders. Officially, the killer was Mario Pepper, in whose home was found Martha Wayne's necklace and who was shot by Bullock when he tried to kill Jim, who wanted to interrogate him about it. In fact, Pepper, while a criminal, was framed (with the necklace being a copy), and when Jim learned about it he swore to find the real killer.
* ''Series/{{Supernatural}}'': The detective in [[Recap/SupernaturalS01E05BloodyMary "Bloody Mary" (S01, E05)]] has the Mary Worthingtin murder. Dean references the trope in [[Recap/SupernaturalS02E09Croatoan "Croatoan" (S02, E09)]], saying that he "feel[s] like this is the one that got away".

[[folder:Video Games]]
* ''Franchise/AceAttorney'':
** The original trilogy has the DL-6 Incident: [[spoiler:the murder of Miles Edgeworth's father in the district courthouse and the spark that sets into motion the rest of the franchise]]. The DS UpdatedRerelease included an extra case and within was the SL-9 Incident: [[spoiler:the murder of a prominent prosecutor by a serial killer that was in fact a staged incident conceived by the future chief of police]].
** ''Apollo Justice'' has [[spoiler:the case where Phoenix got disbarred]].
** There's also the KG-8 case in ''VisualNovel/AceAttorneyInvestigations''. The twist is that it actually did get solved [[spoiler:(Manny Coachen did it)]] but the case was dismissed due to lack of evidence. [[spoiler:This is what led Faraday, Badd, and Yew to form the Yatagarasu]].
** The sequel, ''Gyakuten Kenji 2'', introduces the IS-7 incident, [[spoiler:which was the final case Gregory Edgeworth worked on and brought about Manfred von Karma's only penalty in court. Miles Edgeworth would discover the truth behind the incident 18 years later, and how it ties into the motives of the game's BigBad]]. There is also the SS-9 Incident; [[spoiler:the case where Shi-Long Lang's family name fell into disgrace]].
** ''VisualNovel/PhoenixWrightAceAttorneyDualDestinies'' has the UR-1 Incident, where [[spoiler:Simon Blackquill was convicted of murdering his psychology mentor. In reality he did nothing wrong, but the situation painted a bad picture for a young Athena Cykes and he took the fall to protect her]]. The implications of this case were so strong that it thrust the legal system into a period of distrust by the public and unscrupulous lawyering tactics on both sides of the courtroom ran rampant.
* In ''VideoGame/{{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 [[spoiler:the hit-and-run that killed his wife]].
* In ''VideoGame/{{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 Lucas committed and that Carla investigates follows the same pattern. Reeks of an AncientConspiracy? [[spoiler:There is one]].
* Episode 4 of ''VideoGame/StrongBadsCoolGameForAttractivePeople'', "Dangeresque 3: The Criminal Projective", features a ShowWithinAShow example: Dangeresque's father was driven mad by his failure to solve a kidnapping case, a plot point that seems to be Strong Bad's excuse to fit [[CrazyHomelessPeople Senor Cardgage]] into his home movie.
* In ''VideoGame/LANoire'', Phelps actually solves a real life ThatOneCase, the infamous [[spoiler:Black Dahlia murder]]. But you are informed that due to the killer's family connections, he can never reveal the truth.