Signal

A radio that bridges time

The premise is simple: a profiler in the present and a detective in the past are connected through a battered walkie-talkie. It looks like a time-travel story, but the device is less about fantasy than about letting you see a single case through two eras at once. The show cuts between how a choice made in the past leaves traces on an investigation today, and how something learned today lands on the detective back then. It is a crime procedural, yet the whole way through you carry the question of what might have happened if someone had chosen differently.

How it treats cold cases

Rather than the thrill of catching a culprit, the show focuses on what years without answers do to victims' families and to the people who worked the case. Some episodes recall real unsolved cases that left deep scars in Korean society, so there are moments that are hard to watch as pure entertainment. Each case runs across several episodes, which makes following the rhythm of clues piling up and falling into place a large part of the appeal, and the tone shifts each time a new case begins.

The makers and the cast

The writer is Kim Eun-hee, who wrote the forensic drama Sign and the presidential-security thriller Three Days, and who later became known for Kingdom, which blends a period drama with zombies. Her habit of respecting genre rules while slipping in social questions is clear here too. The director is Kim Won-seok of Misaeng, whose patience with faces and silences fits the tension of a crime story. Lee Je-hoon plays the present-day profiler, Cho Jin-woong the detective in the past, and Kim Hye-soo a detective who appears in both eras.

• Aired: tvN, 2016, 16 episodes
• Writer: Kim Eun-hee (Sign, Three Days)
• Director: Kim Won-seok (Misaeng)
• Cast: Lee Je-hoon, Kim Hye-soo, Cho Jin-woong

Telling the two eras apart

Because the story moves back and forth so often, it can be confusing at first to know which period a scene is in. Props, street scenery, whether anyone has a mobile phone, and the characters' hair and clothes are the surest signals. Many locations reappear in both eras, so comparing how a place has changed sharpens your sense of time in the story. Pay close attention for the first few episodes and it soon becomes second nature. The moments when the radio crackles to life are usually where the two eras meet, so watching closely just before and after them keeps you from losing the thread.

Tips for first-time viewers

Cases do not wrap up neatly each episode; they stretch over several, so watching a few in a row works better than dipping in now and then. Plenty of names come up, but at first it is enough to keep track of how the three leads relate to one another.

• Watch the first two episodes back to back
• Notice the props that signal past or present
• Take a break where one case wraps up
• Save any online searching until you have finished

Worth knowing before you start

Since it deals with murders and disappearances, there are a fair number of crime scenes and tense sequences. Material that recalls real cases can weigh on you, so if you are sensitive to this kind of story, check the age rating and episode guidance first. The investigations and legal procedures on screen are also compressed for the sake of the story and do not mirror how real investigations work. If a run of episodes feels heavy, taking a day off at the point where one case ends is a perfectly good way to watch.

Watching alongside the Japanese remake

A Japanese remake was made in 2018. Comparing how the same premise was adapted to each country's period details and police structure is an enjoyable extra. Watching the original first shows you the skeleton of the story; watching the remake first makes it clearer which memories of Korean society the original leans on. Whichever order you choose, look for the scenes where the two versions answer the same question at different temperatures.

Who it suits

It suits viewers who like mysteries that stay with one case and piece the clues together, and those who want genre drama that raises social questions. If you enjoyed Kim Won-seok's other work, such as Misaeng or My Mister, you will likely recognise the way he frames emotion. If you are after something to leave on in the background, though, go in knowing that this one asks for real attention.