The Witness for the Prosecution unmasks its secrets in a dark and painful concluding episode

Yes, Andrea Riseborough's Romaine Heilger, the seemingly aggrieved common law wife of the suspect Leonard Vole, was in cahoots with him all along. The discrediting of her story in the witness stand was all part of an elaborate plot to get him off the hook of the murder of Emily French, allowing the two of them to live in luxury once she did time for perjury.

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As she tells Toby Jones’ flabbergasted solicitor Mayhew at the end: “What jury would believe a besotted woman? She’d say night is day to save her man. But a vicious scheming bitch lying through her foreign teeth to put the noose round his neck… they would disbelieve her and find him innocent wouldn’t they?”

It is an ingenious plot twist – known to many Christie afficionados – but here given a few smart thematic manipulations by scriptwriter Sarah Phelps.

Many of the key details remain faithful to Agatha Christie's short story, although the Queen of Crime herself amended the yarn when she wrote the stage play. Romaine (called Christine in that version) stabs Leonard in the courtroom when she discovers that he had a mistress along along. It is her only story where the killer gets away with it – and on stage she couldn’t bear to see that repeated it seems.

But here in the TV version, the killers get off once again sending Emily French’s devoted maid Janet (Monica Dolan) to the gallows out of her wits, repeating lines she would have used in her working life before the noose was placed round her neck. But there is a slightly sour touch for Vole when he wonders what may happen if Romaine gets tired of him. "Well don't get tiresome then," she says suggesting their married life may not be entirely straightforward.

Scriptwriter Sarah Phelps' major achievement is to make the First World War the emotional lynchpin of the whole saga.

Mayhew, as Vole and Romaine pointed, out was desperate to save Vole to exculpate his own guilt about his son who died in the trenches.

“I’m going to save him, I’m going to save us.” he told his wife before the trial. The him is Vole - but clearly an unconscious allusion to the son who is beyond being saved.

And in achieving the former, he fails on the latter. Nothing he could do could save his marriage. His wife could never forgive him for allowing their son to die and she can never love him. Mayhew is the architect of his own misfortune. He wanted to be relieved of his own guilt – but he couldn’t see Vole for the man he is.

As Romaine tells Mayhew: "It's what happens when you butcher the young. When you cheat us, you lie to us, you expect us to be grateful."

Vole was not going to return from the war and accept his lot, sacked for dropping a tray while working as a waiter. He wanted money and the good life enjoyed by the kind of people who sent him to the front. And get got it. But the realisation proves too much for Mayhew who kills himself at the close.

In the end this was a story of many loves - of Emily French's love for Vole, of Mayhew's unrequited love for his wife, even Janet's love for Emily. Pole star of course was the passion between Vole and Romaine which burns far too brightly. This was really compelling stuff.