GRIZZLED French detective Julien Baptiste is the unlikely hero of the BBC’s latest hit Sunday night crime drama.

The troubled investigator, who we first saw in 2014 series The Missing, has captivated viewers while doggedly trawling Amsterdam’s seedy underworld after a sex worker goes missing.

And today we can reveal the silver fox character, played by Tchéky Karyo, is based on real-life policeman Jean-Francois Abgrall, who over three years showed a similarly determined approach to catching Francis Heaulme, France’s worst modern-day serial killer. It is feared he murdered 50 men, women and children.

Abgrall is flattered that he is the inspiration for TV detective Baptiste — but he considers himself to be “much cooler”.

He said: “In actual fact we’re not very similar, I find him a bit tortured and haunted. I’m fairly passionate about what I do, but I still manage to stay detached from it all.

“The TV character is intense and very much into it all the time.

“However, Baptiste’s detective skills are very much drawn from the way I work myself. I never put hypothetical theories forward. I demonstrate a crime scientifically.

“Baptiste is a man who never gives up, he will go to the ends of the earth to find the truth.”

Abgrall became the inspiration for Baptiste after he appeared in a 2008 British TV documentary Dance With A Serial Killer made by Nigel Williams.

Nigel’s screenwriter sons Harry and Jack were fascinated by the handsome star of their father’s film and when the brothers wrote the script for BBC thriller The Missing, they based their fictional French detective on Abgrall, a police officer who prefers to work alone.

Abgrall may have mixed feelings about his apparent portrayal on screen, but he is impressed by Baptiste’s depiction of crime.

He said: “It’s a very humane take on police work. The characters are strong and in touch with the psychology of people, which is quite different from a classic detective film genre.

“Beyond the facts of a crime case, like a disappearance for example, you really get into the heart of how human beings operate.

“These series are proof that crime can be a huge deal. There is not just one victim, but many victims, such as the family of a person who has been murdered.”

In Baptiste, the French detective says: “Most of my life I have worked for people who are missing.”

Like the TV detective, Abgrall has been drawn to investigating people who disappear without trace in France every year.

He spent three years working on such cases to uncover the grizzly truth about serial killer Heaulme.

Abgrall’s introduction to the case came when, on May 14, 1989, he was called to the crime scene after Aline Peres, a nursing assistant, was stabbed to death on a crowded beach without anyone noticing.

The murder was coldly efficient — and Abgrall immediately knew he was dealing with no ordinary killer.

Her attacker had knifed her with precision as she sunbathed behind rocks at lunchtime. To stop her crying out, the killer slit his helpless victim’s throat before pushing his knife deep into her, in her heart and spine.

The murder on Moulin Blanc beach, near Brest, north-west France, seemed senseless. There was no reason why anyone would kill this 49-year-old mum of two who had no money in the bank and no enemies.

Abgrall wasn’t to know then that he would spend the next three years locked in a deadly battle of wits with the most prolific and dangerous serial killer in modern French history, nor that he would uncover a blood-stained trail of nearly 50 murders throughout France.

Heaulme, a 6ft 3in, stick-thin drifter, had been staying in a hostel for the homeless near the beach where Aline died.

Abgrall had a gut feeling Heaulme could be the killer and met his suspect a month after the murder.

Abgrall, who turns 60 this month, says: “As soon as I saw and heard this man, his dangerousness and violence were obvious to me. After hours of questioning, I was convinced he was the murderer.”

During the interview, Heaulme claimed to have been in the Territorial Army, where he’d learned to use a combat knife.

He told Abgrall: “To take out a guard you have to surprise him from behind. You use your left hand to raise his head. With your right hand you stab him in the carotid artery, then in the heart and one last time in the spine.”

This described exactly the way Aline Peres was murdered. But Heaulme could not possibly be her killer because he had a cast-iron alibi — he was 50 miles away in hospital and medical records proved it.

Two months later there was another apparently motiveless murder near a homeless hostel at Avignon, in the South of France.

Again, Heaulme was in the area at the time farm mechanic Jean-Joseph Clément, 60, was bludgeoned to death and his body left in a back street.

Abgrall says: “There too, at the appointed hour, Heaulme had an identical alibi. He was in a hospital in Marseille. Luck has its limits. Twice, that’s a lot of luck.”

With no support from his superiors, the diligent detective began to piece together his prime suspect’s extraordinary killing spree, murdering men, women and children.

Abgrall said: “He had no criminal record and was scrupulous in ensuring he was seen to be living within the law. He often took refuge in hospitals — 85 times in five years.”

Gradually, the policeman demolished the serial killer’s ingenious “perfect” alibi.

Abgrall tracked down and interviewed the man in the next bed to Heaulme, who revealed on the day Aline Peres died the suspect was not in the hospital.

The cop discovered that rather than report patients missing and to avoid creating extra paperwork, nurses recorded a made-up temperature, making it look like Heaulme was in hospital.

Abgrall travelled thousands of miles across France, visiting many of the 400 towns and villages where Heaulme had stayed.

Eventually, in January 1992, nearly three years after Aline’s murder, Abgrall had enough evidence to finally bring the serial killer to justice.

He made a final journey to Alsace, near the German border, to arrest Heaulme, two days after he had committed his final murder, killing pensioner Jean Remy in the Channel port of Boulogne.

Heaulme, now 60, is behind bars, serving life for 12 murders.

He last appeared in court two years ago, when he was convicted of killing two eight-year-old boys Cyril Beining and Alexandre Beckrich.

Abgrall, who wrote best-selling book Inside The Mind Of A Serial Killer, retired from the police but is still tracking down murderers as a private investigator.

And just like Julien Baptiste, his wife of nearly 40 years, Annie, does not enjoy her husband’s work.

He says: “She sees me doing interviews at stressful times. I know crime fascinates the public and it’s good to show people what can really happen.

“Murder shatters entire families and creates chaos.”