A cold case is a criminal investigation that has been rendered inactive and unsolved due to a lack of evidence, witnesses, or suspects to form a solid lead. When new evidence does show up or new technology allows for better examination of old evidence, it's a long, difficult, and painful process to peel back the layers of dust covering it and try to put the new lead into context with what's already known about the case, and where that may lead, no one knows.
There are several TV series that revolve around the solving of cold cases. The reopening of an old case may also be used as a one-time plot in a series that usually focuses on fresh investigations. If it's an old case that the protagonist worked on when it was still fresh, this can overlap with That One Case and My Greatest Second Chance.
In Real Life, the statute of limitations impose a limit on this: after a certain time period a case goes officially cold and no legal actions can be initiated anymore. There are practical reasons for this (evidence may be lost or compromised, witnesses may not remember details all that well anymore, and bureaucracy would be a nightmare if dead-end cases remained open indefinitely), and the specific period depends on the local laws. It also depends on the crime committed, with the gravest ones likely having no such limits. The plots may deal with this in three ways. First, to simply ignore it: all that it takes is the protagonists being willing to do it. Second, use it as a time limit: the statute of limitations will be reached tomorrow at 0:00, so we have just one day to solve the old case. And third, the case may have already reached it, the protagonist investigates it just for the heck of it, but then the evidence reveals that there was a higher crime committed: one with no statute of limitations, or that is still within time to initiate legal actions. That, or the criminal turns out to be someone from the cast or someone the investigator knows; so that there can be drama even if the case stays closed despite the investigation.
A related trope is Extremely Cold Case, where a detective applies their crime-solving skills to a historical mystery so old everybody involved is long dead.
- In Kurokochi, the main characters Shingo Seike and Kenta Kurokochi often discover the true story behind well-known cases of post-war Japan, such as a the Teigin Bank robbery in which the staff were all poisoned by cyanide, not because of the money but because they served as guinea pigs for a far-right conspiracy or the 300 million yen robbery in which an unknown young man in uniform stole a company's bonuses for its employees and whose culprit was caught bur covered by a burgeoning police conspiracy as the young man in question was a commissioner's son. All of these cases have links to current cases they must solve, as they help Seike and Kurokochi investigate the current motives of their suspects.
- LAPD detective Harry Bosch investigates cold cases in several of his novels, and in the later novels of the series leads a unit specializing in "open-unsolved" cases. He manages to mix this with It's Personal in The Last Coyote when he investigates the murder of his own mother, strangled and left in a dumpster when Harry was ten.
- Leif G.W. Persson's Backstrom crime novels: The last novel in the series, The Dying Detective (Den döende Detektiven), sees very senior copper, Lars-Martin Johansson, recuperating after a stroke, reviewing a cold case which was bungled a quarter-century before by Bäckström. As his last piece of active police work, Johansson undoes the damage caused by Bäckström and solves an especially horrible murder. But the effects of his illness are too much and he relapses and dies.
- In Le Testament de Sherlock Holmes by Bob Garcia, Sherlock Holmes investigates a string of gruesome murders. Due to his cocaine addiction, he basically fails, with the case being closed after about twenty people either murdered, or executed despite being innocent. After giving up cocaine, he revisits the case, and solves it rather quickly despite all the time that passed.
- Agatha Christie:
- Hercule Poirot's Five Little Pigs has the Belgian detective revisit a 16-year-old case when the daughter of the (now deceased) convict requested that he clear her mother's name.
- In Ordeal by Innocence, a two year old case gets reopened when an eyewitness suddenly show up to testify the alibi for the murderer, who has since passed away in prison. The supposed murderer was then granted free pardon, and the police was forced to re-investigate and find the true murderer, to the distress of the remaining suspects.
- Troubled Blood: For the first time private detective Cormoran Strike takes on a cold case, as a woman engages Strike to look for her mother, who disappeared from a London street in 1974, 39 years ago. Cormoran, being an honorable sort, takes the case only after candidly admitting that the odds of finding out what happened to the missing woman are slim. (Of course, he does.)
- On Blindspot, the appearance of Jane Doe reopens an old case with a personal connection to FBI Agent Kurt Weller. Taylor Shaw was a childhood friend of Weller's and her disappearance wrecked his life. Weller's father was the main suspect in the case but there was not enough evidence for police to file charges. However, the family remained under a cloud of suspicion with most of their neighbors believing that Weller's father was a child-killer. When Jane shows up, DNA evidence shows that she is a grown up Taylor Shaw which causes everyone to question what really happened when she disappeared all those years ago. The evidence was faked. Weller's father accidentally killed Taylor and buried her body in the woods.
- On Brooklyn Nine-Nine, a few episodes have the characters take up an old case they had worked on but was never solved.
- In Unsolvable, Jake and Terry take on a case they worked on several years before about a man who died in his boat. It turns out that the killer was the supposed victim, who killed his lover's husband and then set the boat on fire to fake his death.
- In Terry's Kitties, Jake helps Terry solve his first case, which he never managed to solve because the main suspect was wheelchair-bound. Terry had been right on the dot - as the suspect was faking his disability.
- The murder of Kate Beckett's mother was a cold case and trying to find the truth motivated Kate to become a police officer. When Castle finds out about it, he tries to re-investigate the case, which causes Kate to become very angry with him. She has tried to solve the case many times over the years and each failure left her extremely bitter and depressed. Castle investigates anyway and a forensic expert he hires finds evidence that sheds new light on the case. Over the subsequent seasons, Castle and Beckett find evidence of a criminal conspiracy that the murder was part of and try to bring everyone responsible to justice.
- Episode "That '70s Show": During construction a body is discovered inside a support column of a building. It is revealed to be that of a prominent mobster who disappeared during The '70s. The police always suspected that he was murdered but without a body they had no evidence to go on. Castle and Beckett reopen the case but the investigation is hindered by the fact that the main witness in the case had a mental breakdown and believes that he is still living in the '70s.
- Cold Case is about a Philadelphia Police Department division that specializes in investigating cold cases.
- Actually, some episodes suggest that it's only the show that specializes in these investigations. The characters seem to be part of a regular homicide unit, but all of their investigations into fresh homicides happen off-screen.
- Cold Squad is about a Vancouver Police Department division that specializes in investigating cold cases.
- One episode of Crossing Jordan had Jordan's father call together the staff at the medical examiner's office for an exercise at solving a cold case. After playing out the scenario, they realize that modern technology could easily solve the case. They run the evidence through their equipment and come up with a culprit. They then set out to apprehend the murderer only for the elder Cavanaugh to inform them that the murderer died a few months earlier.
- Death in Paradise:
- In "In the Footsteps of a Killer", DI Mooney reopens a 7 year old murder case when a witness comes forward and provides an alibi for the woman originally convicted of the crime (who subsequently died in prison). The witness had left the island before the crime was discovered, and never returned until now.
- In "Melodies of Murder", DI Mooney has to reopen a 30 year old cold murder—originally investigated by Commissioner Patterson—when the husband of the victim announces that he has new evidence regarding the murder, and is promptly murdered himself.
- Once on Frasier, Martin dug up a murder case he was unable to solve as a police officer. He tries looking into it with the help of Daphne, Frasier, and Niles. While he's out of the room, Frasier proposes an unusual but plausible solution to the murder and suggests rearranging the crime scene photos to inspire Martin to come to the same conclusion. It works and Martin manages to solve the murder but the actual solution is not the one Frasier came up with.
- On Golden Boy Detective Owen keeps re-investigating an old cold case which was actually his first case as a detective. The murder occurred on 9/11 and the investigation was interrupted when the planes hit the World Trade Center. When he was able to return to the case, the trail has gone cold and evidence was lost. It became That One Case for him. The case is finally solved when a witness discovers that he is dying and decides to come clean about what really happened.
- Inspector George Gently: In "Gently Liberated", the discovery of the body of a man who went missing in 1962 causes Gently to reopen the case, as his wife had been convicted of his murder on flimsy evidence despite the lack of a body.
- On Justified Mob boss Theo Tonin killed a FBI informant but the murder could not be tied to him because the only witness, drug smuggler Drew Thompson, died when he jumped out of a plane and his parachute failed. Twenty years later, the US Marshals discover that Drew faked his own death and the dead body belonged to someone Drew killed. The Marshals reopen the case and try to track down Drew. They run into major problems since they have no photos of Drew and only a vague description based on two decade old memories.
- Done frequently in the Law & Order franchise. Such investigations are usually, but not always, kicked off with a dying declaration.
- One episode was about investigating the death of the world's oldest man, which happens to be in the same neighborhood as a hit-and-run which Stottlemeyer was unable to solve. The oldest man was the subject of a documentary directed by Mrs. Stottlemeyer, and Lt. Sottlemeyer's refusal to take the case seriously causes a rift between them. Monk suggests watching her documentary on the old man to make her happy. While doing so, they spot a clue that not only solves the old man's murder, but the hit-and-run as well.
- The two-part series finale was about Adrian Monk finally solving the mystery of his late wife Trudy's murder.
- Murdoch Mysteries:
- "Confederate Treasure" dates back to the U.S. Civil War in the 1860s. Skeletal human remains in an old suit, with an old pistol in one pocket and a document bearing the signature of Canada's first prime minister suggest the date. The newspaper archives indicate a Canadian minister went missing in the 1860s, but, well, there was a war on in the neighboring country...
- "Unfinished Business" has Murdoch share a recorded deathbed confession of murder with Dr. Ogden. He's puzzled that the confession details don't match the data Dr. Grace gathered from the corpse he and his colleagues found using the dead man's directions. Dr. Ogden recognizes the details of the confession as matching a case she and Murdoch worked on eight years earlier, one in which Murdoch investigated the husband of the murder victim but had to drop the matter for lack of evidence.
- "What Lies Buried" involves a corpse that dates back over twenty years before the current timeline (1881, when the episode is set circa 1902). Workmen digging in the station house basement turn up the remains, and the investigation leads to a constable who disappeared and the unsolved murder of a rent boy.
- New Tricks is about a Metropolitan Police division that specializes in investigating cold cases.
- Signal is about a Korean police division that specializes in investigating cold cases, with a Write Back to the Future twist.
- Unforgotten revolves around two London detective and the cold cases they investigate together.
- Vera: "Telling Tales" opens with a convicted murderer who is ten years into a life sentence committing suicide. The publicity brings to light an alibi that had been ignored by the original investigators which proved her innocence. Vera and her team have to re-investigate the original murder.
- Waking the Dead is about a Metropolitan Police division that specializes in investigating cold cases.
- Occasionally, some of the cases on Without a Trace were this instead of the typical "hot" ones. Surprisingly, despite the length of time since their disappearance, the victim was sometimes found alive.
- In the first Phoenix Wright: Ace Attorney game, the climactic chapter involves a new development that leads to Phoenix reinvestigating the 15-year-old murder of Miles Edgeworth's father, and solving the case on the day before the statute of limitations is due to run out on the crime.