There’s nothing the British enjoy more than a cozy little murder. Whether on Sunday night or Christmas Eve, Britons from all walks of life luxuriate in watching a wealthy dowager sip a poisoned cocktail, or seeing a spinster take a knitting needle to the throat. For decades these grisly events have been reliably provided by the Agatha Christie television adaptations that have become a highlight of British national holidays, regularly drawing millions of viewers. Yet recently this blood-flecked tradition has been shaken to its very foundations.

The person doing the shaking is Sarah Phelps, the writer of a quintet of mini-series—And Then There Were None, The Witness for the Prosecution, Ordeal by Innocence, The A.B.C. Murders, and, on Amazon Prime this month, The Pale Horse—that have roiled Christie traditionalists and casual TV viewers alike. On the face of it the shows would hardly appear to be controversial. They’re beautifully filmed and populated by such fine character actors as John Malkovich, Bill Nighy, and Toby Jones. However, Phelps’s ruthless reconstruction of Christie’s work—her insertion of sex scenes, drug-taking, and language most foul—has caused a national crisis on the order of Brexit.

A scene from Phelps’s 2015 mini-series And Then There Were None.

“Agatha Christie and the Mysterious F-Bomb,” read the front page of the Daily Star, “Poirot Outrage” led the Daily Mail, while on Twitter one user wrote, “So there was a murder and the victim was Agatha Christie’s book.” To make matters worse was Phelps’s confession that she had never read any of Christie’s books before she wrote her scripts. “I thought she was safe; I thought she was cozy. I thought there was a sort of quality to her that wasn’t me,” Phelps says from her home in what she calls the “Witch Hunter Corridor” of East Anglia. Indeed, when offered the chance to adapt And Then There Were None, Phelps dragged her heels. Then she started reading. “It knocked me sideways with its savagery,” Phelps says of Christie’s most famous murder mystery. “It’s a merciless book, and I remember thinking, ‘This is Aeschylus; this is the Greeks.’ The absolute horror of knowing that whatever you try and do, however you beg, however you plead, however you wriggle out of it, there is no mitigation, there is no mercy. Your oblivion is coming and it is white-eyed, and it is remorseless and it does not blink.”

Phelps’s 2015 BBC mini-series And Then There Were None, which starred Miranda Richardson and Sam Neill, among others, was dark, gruesome, and infused with a stark realism that made the arch plot seem horribly authentic. It shook viewers out of their comfort zones and forced them to admit that perhaps a vicious murder wasn’t so cozy after all. “Because I had no familiarity with her, I was able to be really shocked,” says Phelps. “She’s such a global brand you forget she’s a writer, you forget she has a mind like a steel trap, and she’s got something to say.”

“It knocked me sideways with its savagery,” Phelps says of Christie’s most famous murder mystery. “It’s a merciless book.”

Phelps, who grew up in the small town of Molesey in Surrey, left school at 16 and worked in a succession of jobs in pubs and horse stables. While employed as a dresser at the Royal Shakespeare Company, she wrote a play about the “brutal life” of telemarketers. That led to a writing gig on Eastenders, Britain’s grittiest soap opera. Her reputation grew and she was asked to adapt Dickens (Oliver Twist and Great Expectations) and Rowling (The Casual Vacancy). Her move into crime writing (besides Christie, Tana French’s Dublin Murders) was not conscious. “It’s less about crime than people twisting on the pin of what they’ve done,” she says. It’s this willingness to stress Christie’s characters as much as her twisted plots that truly sets her adaptations apart.

Rufus Sewell as Mark Easterbrook in Phelps’s final Christie adaptation, The Pale Horse, now streaming on Amazon Prime.

What we know of Miss Marple’s and Hercule Poirot’s inner lives could fit on the back of an arsenic-laced postage stamp. But Phelps scoured the books for the tiniest of details with which to draw out full-blooded characters. “You have to dig around in her,” she says of Christie. “I think she plants clues to see if you’re paying attention. You can have a jolly time and read it in a rainy afternoon, or you can spend some time and go, ‘What is that discordancy? That strange detail? What is it doing there?’ I pursue those like a bloodhound to their logical conclusions.” In Phelps’s hands, a character’s “little dry cough” becomes the result of his having been gassed in the First World War, while his barely mentioned wife becomes a silent woman wracked with grief. Drawing out such ciphers and placing the murder mysteries in real historical time allowed Phelps “to create a portrait of the blood-soaked 20th century through the eyes of cozy, little, stealthy, subversive Agatha Christie.”

The Pale Horse, based on the 1961 book, will be Phelps’s final Christie adaptation. It stars the magnificently vulpine Rufus Sewell as a playboy turned reluctant detective stumbling into a murder plot that seems to involve the use of magical curses. But while it stays relatively faithful to the book’s plot, it is also suffused with Phelps’s own preoccupations. A fan of folk horror, Phelps drew on her knowledge of strange harvest-festival rituals and inserted them into Christie’s clockwork story. The result mixes Mad Men–era sexual shenanigans with the pagan horror of The Wicker Man and seems as much a repudiation of Britain’s own clichés of rural life as it is of Christie’s reputation as a genteel crime novelist. “If you’re not going to turn those stories upside down and shake them, then all you’re doing is writing to the brand and not writing to the author. There’s a whip-smart intelligence in there, a flinty-eyed bisector of humanity.”

George Pendle is a Contributing Editor for Air Mail