The story revolves around Lt. Park Hae-young, a profiler who hates cops after they ignored what he had to say regarding two criminal cases during his childhood. One of them was the kidnapping and murder of a classmate, an event that he witnessed and tried to inform the authorities of, but was ignored. As the statute of limitations of the kidnapping case comes close, Lt. Park discovers an old walkie-talkie, getting a message from a detective who gives crucial information with regard to catching the culprit.
After this incident, in response to the protest against the statute of limitations, the Korean government lifts that policy and a cold case squad is formed.
As he works his way around these mysteries, Lt. Park realizes that on the other end of the walkie talkie is a detective from the past named Lee Jae-han, who is missing in the present. After initial disbelief over the whole situation, the two work together in solving said cold cases by giving each other information from the past and present.
The cold case squad in the present, led by Detective Lee's junior, Cha Soo-hyun, earns recognition and face difficulties solving many unsolved crimes.
A Japanese version based on the show has aired on Fuji TV under the same title.
This show provides examples of:
- Action Girl: Detective Cha Soo-hyun. In a fight, she's more than capable of holding her own.
- Adorkable: Jae-han when he's trying to summon the courage to approach Won-kyung.
- Adult Fear: Your child being abducted and murdered, the perpetrator never being brought to justice. Watching your child die and being unable to do anything.
- Arc Words: "Never give up."
- Bolivian Army Ending
- Bittersweet Ending: Several storylines have a somber end even when the case is solved.
- In the first arc, the culprit of the kidnapping case is arrested for the murder of a man who knew of her crime, but the statute of limitations for the abducted girl's death has passed and can't be prosecuted, much to the despair of the mother. These events lead to a backlash against the statute of limitations, causing the government to repeal the policy on unsolved murders, leading the formation of the Cold Case Squad.
- Lee Jae-han manages to clear the name of an ex-con who was framed by the real culprit, preventing Cha Soo-hyun from being indirectly killed by him in the present. The ex-con however, is arrested anyway for murdering a man he blames for his daughter's death in a bus explosion and dies in prison.
- Big Brother Worship: Hae-young idolized his brother Sun-Woo to a big extent. The flashbacks show us that Sun-woo was a really good brother and person and completely deserving of Hae-young's love and affection.
- Butterfly of Doom: Both Jae-han and Hae Young learn the hard way that actions have consequences, especially when it comes to the past.
- After learning of the location of the next crime scene from a serial killer from Lt. Park, Detective Lee is able to save the victim, sees the culprit and catches him...or so he thinks. While the victim managed to survive and have a family, the person that Detective Lee arrests (who happened to be wearing the same outfit as the killer) dies in custody when the chief becomes too aggressive. The murders of the next two victims also occur much sooner, with another person killed with the same MO in the present time.
- After learning that a burglary case would remain unsolved in the present time, Detective Lee asks for some tips, which Lt. Park gives. He ends up arresting an ex-con that he befriended, but as a result, the ex-con's daughter, who saw Lee as an uncle, dies in a bus accident.
- In a case about serial killings with the victims asphyxiated, which has nine victims, Detective Lee manages to catch the culprit sooner despite Lt. Park refusing to give any information. This trope is subverted in that nothing wrong seems to be apparent, with future victims being still alive in the present, though Lt. Park thinks something bad could have happened and he just doesn't know what it is.
- Chekhov's Gun: Lee Jae-han's notebook.
- Cowboy Cop: Once Jae-han realises that most of the cops he knows cannot be trusted, he begins to take his own initiatives.
- Dead All Along: When he first talked to Detective Lee, Lt. Park searched for any members of the police with his name, only finding people with the same name and a file that says he's missing. After realizing the walkie-talkie communicates with the past, he does some digging and later finds his remains underneath a house. It is later revealed that Detective Lee was murdered by a crooked cop for having evidence of Lt. Park's brother being murdered by his boss.
- Disposable Woman: The majority of murder victims are mainly women, one of them being a child in the beginning.
- Dirty Cop: Unfortunately, the series is liberally peppered with them. The biggest offender though is Kim Bum-Joo who rose to his current position by engaging in bribery, corruption and even murder.
- The Dog Bites Back: Sick of being his boss' whipping boy and haunted by killing Lee Jae-han, Ahn Chi-soo hides that Park Hae-young is Sun-woo's little brother and offers to tell him everything.
- He Knows Too Much: The reason Lee Jae-han met his end in the original timeline.
- Heartbroken Badass: Jae-han, in a manner of speaking. Won-kyung's death spurs Jae-han into become a detective who will not rest until victims receive justice.
- My Greatest Failure: The four cold cases that the Cold Case squad is tasked with solving are all cases that Lee Jae Han considered his greatest failures either because he could not catch the culprit right away or because he was he was thwarted by other factors.
- Near-Death Experience: All the three leads have near death experiences during the course of the drama. Cha Soo-hyun almost becomes the victim of a serial killer. However, she is able to escape in time and is luckily found by Jae-han. Park Hae-young takes a bullet for Cha Soo-hyun and comes close to losing his life. Lee Jae-han survives an attempt on his life by Kim Sung-bum, Kim Bum-joo and Ahn Chi-soo. Luckily for him, he's able to call his team for back-up and they arrive in the nick of time.
- One-Word Title
- Police are Useless: Lt. Park's reason for his grudge against them.
- Portal to the Past: Much like Frequency, it can only transmit sound.
- Posthumous Sibling: Park Sun-Woo. The mystery of his death plays a big role in the second half of the drama.
- Promotion to Parent: Sun-woo took care of Hae-young like a parent would, because their parents were often not around.
- Rape as Drama:
- A high school gang rape case is a major part of Lt. Park's backstory, as his older brother was wrongfully convicted of instigating an assault on a classmate.
- A woman who faked her death reveals that she was a victim of this.
- Revisiting the Cold Case: The premise of the series.
- Ripped from the Headlines: Some of the cases have similarities to real life cases in Korea, which were also dramatized in other media:
- The kidnapping of Kim Young-joo, Lt. Park's classmate, seems to be inspired by a real-life abduction of nine-year-old boy in 1991. Despite 87 ransom calls from the kidnapper to the child's parents asking for ₩70 million (US$75,000), Lee's dead body was found in a ditch near his home 44 days after he went missing; the killer was never caught, and the 15-year statute of limitations on the case expired in 2006. This incident was the basis for the Korean film Voice of a Murderer.
- The cold case squad's first assignment after its formation is basically the Hwangseong murders with a different case name, which Memories of Murder was based on. It is also chronologically Lee Jae-han's first time communicating with Lt. Park.
- Serial Killer: Two of the cold cases the squad is asked to solve deal with serial killers, one of them already mentioned above.
- A later case reveals that multiple murders by asphyxiation took place in a small town right under the authorities' noses from 1990 to the present.
- Screw the Rules, I Have Money!: A running theme that Lee Jae-han comes up against many times in the past. The perpetrators in two cases are boys who are wealthy and well-connected.
- Spoiled Brat: Many minor characters are rich kids who think they can get away with anything because they have rich fathers backing them.
- The Profiler: Park Hae-young himself. While he is good at deductions, he also stresses that it is not an exact science.
- The Lost Lenore: Won-Kyung for Jae Han. He's never really able to get over it and the fact that the couldn't save her in time forever remains a source of grief.