Viewers are gripped to BBC One's The Serpent, a true crime drama which explores French serial killer Charles Sobhraj.
The show is set in the 1970s and explores Sobhraj's crimes in Asia and the man who set about investigating him, Herman Knippenberg (played by Billy Howle in The Serpent cast).
The Serpent is based on a true story – here's everything you need to know about the real-life events surrounding the BBC drama.
Is The Serpent based on a true story?
Yes, BBC One series The Serpent is based on the case of real-life French serial killer and con-man Charles Sobhraj (nicknamed “The Serpent” and “The Bikini Killer”), who was captured and put on trial in 1976.
Widely considered a psychopath, Sobhraj was the chief suspect in at least a dozen murders of Western travellers that took place in South Asia in the 1970s (he allegedly fostered a deep-seated hatred of "hippies," or as they're sometimes referred to in The Serpent, "longhairs").
As the BBC states: “Posing as a gem dealer, Sobhraj and his girlfriend Marie-Andrée Leclerc travelled across Thailand, Nepal and India in 1975 and 1976, carrying out a spree of crimes on the Asian ‘Hippie Trail’ and becoming the chief suspects in a series of murders of young Western travellers.”
For a time, Sobhraj was Interpol’s most-wanted man, but his capture was thanks in part to the determination of a 30-something junior Dutch diplomat, Herman Knippenberg, whose perspective we follow in BBC One's The Serpent.
Who is Herman Knippenberg? Is he alive?
Herman Knippenberg is a former Dutch diplomat who first became aware of Charles Sobhraj when he was working in Bangkok at the Dutch embassy.
In February 1976, he began looking into the case of two missing Dutch travellers: Henk Bintanja and Cornelia 'Cocky' Hemker.
Knippenberg recalled an article he had read two months earlier about two murdered Australian backpackers, and after some investigatory work he soon realised that the burnt bodies had been misidentified - they were, in fact, the Dutch couple he sought, Bintanja and Hemker.
Knippenberg had unwittingly walked into Sobhraj’s intricate web of crime, and would spend the coming months hunting down the mysterious French 'gem dealer,' who was finally apprehended later that year, in July 1976.
Billy Howle (MotherFatherSon), who plays Knippenberg in The Serpent, spoke to Knippenberg in preparation for the role.
Speaking to RadioTimes.com and other press for The Serpent Q&A, Howle revealed that he had found approaching the script "daunting... ... in terms of there being a real sense of gravity and responsibility, because these are real-life people."
He added that it was "enlightening" speaking to Herman Knippenberg.
"It was a first for me to play someone who is not only real but still alive. You know, I was acutely aware of that responsibility, so at first I was slightly reluctant to speak to Herman straight away because I felt I needed to mine the script and really come to terms with the story that we were trying to tell and do all the practical stuff, the research, and have a better understanding of the landscape of the story, too.
"And then when I spoke to Herman, it was incredibly enlightening... not only [for] the story, but his perception of the story and the events that took place are so far removed from my own experience in life that it wasn't immediately relatable. So there was a lot of groundwork that needed to be done and I was very fortunate in being able to build that bond with Herman quite early on, because he was able to fill in those gaps and increase my understanding of the situation."
He also revealed that when he spoke to Knippenberg about why he had been so motivated to capture Sobhraj, he realised that it had been the diplomat's "moral outrage".
"It's a sort of moral question: what drives a man to do such heinous things as Charles Sobhraj has done? And interestingly, when I spoke to the real Herman, he said, 'Who cares?' And that was quite amazing for me to hear, and I kind of understood straight away what he meant. Well really no, it's not about trying to understand what drives him [Sobhraj] to do that, it's about stopping it. And you know in a sense, how dare another human being do that to innocent people?
"So it's a sort of moral outrage that drives this fastidious investigation that he takes upon himself. It's so unlikely, really, a person in this position doing the job that he was doing, for this to land on his desk, I think it is the moral outrage at the reality of this, the gravity of it, that drives him forward to want to stop it."
Ellie Bamber, who plays Herman's multilingual first wife, Angela, also spoke to her real-life character about the role.
"It's amazing, she's a really intelligent woman, and yeah, she was able to fill in the gaps with everything, all the little details that might not have been there in the script," Bamber said. "She sent me almost a hundred pictures from around the time, so she sent me all the pictures of Herman and her together, Herman at work and her at work, so that was amazing to fill in those visual gaps, I suppose."
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Is Charles Sobhraj still in jail?
Yes, Charles Sobhraj is still alive and currently serving a life sentence behind bars in Nepal, where he has been since 2003.
Prior to his arrest in Nepal, Sobhraj had been in jail in India from 1976 to 1997, after which he returned Paris and enjoyed his notoriety and newfound celebrity.
Sobhraj has previously been depicted on-screen in the 2015 Bollywood movie titled Main Aur Charles, in addition to his upcoming portrayal in The Serpent, in which he is played by Tahar Rahim (who did not speak to the real Sobhraj for the series).
The Serpent will begin on New Year's Day, 1st January at 9pm on BBC One. Looking for something else to watch? Check out our guide to the best TV series on Netflix and best movies on Netflix, or visit our TV Guide.