Image copyright CJ ENM Image caption The drama centres around the unlikely love between a South Korean heiress and a North Korean officer

An implausible love story in which a (literally) high-flying South Korean heiress accidentally paraglides into North Korea, lands on a soldier and falls in love with him has become the latest Korean drama smash hit.

Crash Landing on You is in many ways a typical K-drama romance, but has been widely praised for its well-researched and nuanced portrayal of North Korea, something it achieved by having a real-life North Korean defector on its writing team, as BBC Korean's Subin Kim explains.

With his broad shoulders and thick torso, Kwak Moon-wan has all the appearance of a bodyguard.

That's probably because until 2004, he served with the Supreme Guard Command, the elite security force which protects North Korea's ruling Kim family.

He was so trusted that he was assigned to work overseas too, for a North Korean trade company in Moscow which was bringing in much needed foreign currency.

Only a select few North Koreans are permitted to work outside the country, and to ensure their continued loyalty the leaders have measures in place - Kwak had to leave his wife and son behind in North Korea.

Image copyright Kwak Moon-wan Image caption Kwak Moon-wan studied film directing in Pyongyang

In 2004, he was ordered to return to Pyongyang. During a stopover in Beijing, he found out one of his friends in Moscow had reported to their bosses in Pyongyang what he had said in private conversation.

He knew immediately that what he'd said would cause huge trouble when he got home.

During our conversation in a coffee shop in Seoul, Kwak kept glancing around to check who might be nearby. He speech is straightforward, if not blunt, but Kwak wouldn't repeat to me what he said. He'd only say he had talked about what he saw while escorting members of the Kim family back then.

So he decided to defect. Alone. And he has lived in South Korea without his family ever since.

"I only have one shadow when the sun comes up," Kwak said. "That's my life in South Korea."

After arriving in South Korea, Kwak, like thousands of North Korean defectors, began the process of building a new life. And it took a remarkable twist of fate for Kwak to find his way into the booming world of Korean entertainment.

From defector to film adviser

Before entering the military, Kwak had spent time learning about film. Back in the 1980s, the North Korean film industry was booming, because of then leader Kim Jong Il's well-known love of the art.

At that time, Kwak was about to embark on further education, and he ended up being accepted to study film directing in Pyongyang University of Dramatic and Cinematic Arts.

Image copyright CJ ENM Image caption Despite all the cultural differences, the two can't help falling in love

Shortly after Kwak arrived in South Korea, a famous filmmaker who was working on a North Korea-themed film project approached South Korea's spy agency asking for some advice.

Kwak had just finished his interrogations, part of the resettlement process new defectors go through, in which he'd talked about his film skills.

The agency put him in touch with the filmmaker, who offered him a job at his film company. Kwak accepted it right away.

He went on to work as an adviser and a screenwriter on a number of films and dramas, and in 2018 a former colleague introduced Kwak to Park Ji-eun, the head writer of the drama.

She had come up with an idea of a romantic comedy featuring a North Korean officer and a South Korean heiress, but her lack of intimate knowledge of Northern life was a pressing concern.

Kwak joining the team kick-started the Crash Landing on You project.

Undercover spies and homeless children

The series has become one of the most successful Korean dramas of all time. It tells the story of heiress and businesswoman Yoon Se-ri and North Korean army captain Ri Jeong-hyuk.

While out paragliding one day, Se-ri gets caught up by freak winds, and pushed over the border into North Korea. She is found by the dashing Jeong-hyuk, who instead of turning her in agrees to keep her safe and help her return home. Inevitably, they fall in love.

Kwak's intimate knowledge of how North Korean officials operate meant he was able to contribute ingenious plot devices.

For example, at one point, secret police come across Se-ri hiding in a village. Jeong-hyuk quickly comes up with the line that she is a spy with Division 11, the military unit which works undercover in the South.

That helps explain her Southern accent, her lack of paperwork and her appearance, and gave the character the freedom to explore the village and interact with others, while refusing to answer their questions about her life on security grounds.

Image copyright CJ ENM Image caption Se-ri's cover story helps her blend in with the local North Korean women

Throughout the show, there are depictions of life which could be made credible through the insights of someone like Kwak.

Trains are shown abruptly stopping because of power cuts, homeless children on the streets, and fridges used to store books and clothes instead of food.

Kwak also helped create a subplot in the drama, of another pair of star-crossed lovers from the North and South, Gu Seung-joon and Seo.

After embezzling a huge amount of money from Se-ri's brother, Seung-joon goes on the run, and decides to seek refuge in North Korea.

"North Korea is the only place the Interpol can't reach," Kwak says.

Is it true that North Korea offers protection for wanted criminals in exchange for a hefty amount of money? "It is indeed plausible," Kwak says. "That's all I'd like to say."

Praise from other defectors

Some have accused Crash Landing on You of glamorising North Korea. For example, villagers seem to have plenty of food - but in reality food shortages are a recurring problem.

But these nuanced details are rarely seen by South Koreans, and the drama has even got other defectors excited.

Chun Hyo-jin, who defected when she was 19, said the drama does deviate from reality but that does not dent her enthusiasm for it. Most of her family are now living in the South, and the drama has become a weekly topic.

Image copyright CJ ENM Image caption The two lead actors are stars in South Korea

"Every time it's on air, we call each other on the phone and talk about the drama," Chun says.

"It has made the people interested in North Korea. It gets my friends to ask me about North Korea and I'm really grateful for it."

It's also won praise from people like Sokeel Park, who works with defectors through Liberty in North Korea.

"Its portrayal of various aspects of North Korean society have clearly been thoroughly researched, resulting in the most three-dimensional portrayal of North Korean society of any film or drama to date," he told the BBC.

Media playback is unsupported on your device Media caption Four defectors talk about what life is like in North Korea

"It is refreshing how it portrays various aspects of North Korean society without unnecessarily passing judgement, and shows North Koreans as complex people who are ultimately relatable and even lovable, even if they are culturally different."

It's well known that North Koreans, especially the young ones, are fans of K-drama too - and as Crash Landing on You accurately depicts, a lot of South Korean films and dramas are smuggled into North Korea.

Kwak says he hasn't heard of any North Korean who has watched the drama.

"I'm sure they will be very interested. It's their story. It's about them.

"And I guess that especially North Korean men would feel very grateful to have a handsome guy like Hyun Bin to play one of them," he chuckled.