With Ordeal by Innocence ending on Sunday, its writer tells Nick Curtis why she changed Christie’s twist

FOR the past two weeks the nation has been held in the tight, clammy grip of Ordeal by Innocence, writer Sarah Phelps’s latest three-part exploration of the nation’s dark psyche through the works of Agatha Christie. We have savoured the glamorous, classy cast (Nighy! Chancellor! Treadaway! Tomlinson!) of this fraught family mystery, which concludes on Sunday.

While soaking up the sumptuous period detail we have boggled with the knowledge that 45 minutes of the three one-hour episodes were painstakingly re-shot with Christian Cooke replacing Ed Westwick as Mickey Argyll, one of the adopted children of murdered Rachel, after Westwick was accused of sexual assault (allegations he denies). We’ve f***ing loved the swearing because families, and murder, make you swear and because, as Phelps again puts it, “we are all grown-ups”.

But one thing we may not have noticed is t h a t A n n a C h a n c e l l o r ’s R a c h e l represents the Queen. Or, as Phelps explains, she represents something the Queen represented in 1958, when Christie wrote the story. “It’s after the Second World War, after the A-bomb has been dropped and you know you can scorch a human being to dust and devastate entire cities,” she says. “How would you get over that knowledge? We are in the 1950s and there is a new young queen, a young mother; everyone is happy, they are new Elizabethans in a new Renaissance, going forward. And yet there is all the blood.”

Rachel is a dark echo of the Queen, “a mother who cannot be a mother, who had to adopt. In a way she typifies this idea of glorious womanhood who nurtures and creates stability, a family and a beautiful home. But threaded through the book is terror: she won’t let the children play or climb trees. So there is all the fluttering bunting, having won the war, rationing being lifted, but under it all is this deep, deep, current of paranoia and fear.”

Part of her job was to rescue the story from its “blanket of cosiness”, as she did with previous Christie adaptations for the BBC, And Then There Were None and Witness for the Prosecution. “It’s always about the spirit rather than the letter of the book,” she says. “I try to honour the spirit of the writer. I find something very shocking and subversive in Agatha Christie, and that’s what I’m writing.”

As the great director Billy Wilder noted, Christie was terrific at plot but terrible at character, while Raymond Chandler was the other way around. Phelps says she expands “little throwaway details into character” and has no compunction at altering the plot. “There is a wedding proposal at the end of Ordeal by Innocence, but not in my version,” she says, though we agree to refrain from further spoilers. The book is static, talky, retrospective and light on psychological detail. The backstories she’s written in for the characters (impotence, mental problems, status anxiety) make sense of their actions.

But having adapted Dickens as well as Christie for TV, and more recently