How is THAT Ordeal by Innocence ending different from the Agatha Christie novel?

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The final episode of screenwriter Sarah Philips' Ordeal by Innocence takes a dramatic turn away from the 1958 novel and (SPOILER ALERT) gives us a different killer entirely. It's not housekeeper Kirsten Lindstrom who walloped Rachel over the head and then murdered the meddling Philip, as in the book; instead it's "daddy" Leo Argyll who did the deed.

So how exactly is the TV version different from Christie's original story? Here's how it all plays out:

Did Leo kill Rachel in the book?

In short, no.

In the book, it was Kirsten who killed Rachel. Young "delinquent" Jack was a prolific seducer of older woman and an expert at manipulation, and so he charmed middle-aged housekeeper Kirsten and made her believe he was in love with her. Then, when his adoptive mother refused his requests for cash to pay off his debts, Jack persuaded Kirsten to pop in to her office, steal the money and murder Rachel on his behalf.

Jack's plan was to establish his alibi for Rachel's time of death by hitchhiking into town. But he had the misfortune to be picked up by Dr Arthur Calgary, who failed to come forward to the police in time to save Jack – partly because he'd soon be battling concussion and amnesia, and partly because he was about to head on an Antarctic expedition.

So Jack was arrested with no alibi – and plenty of motive and a wallet full of Rachel's money! He was convicted and died in prison of pneumonia six months into his sentence, and everyone breathed a sigh of relief.

Everyone, that is, except Kirsten. She only realised she'd been fooled when Jack's secret wife Maureen turned up the morning after the murder (yes, Jack was married!) and threw herself on the kindness of the Argylls. Jack, the man Kirsten had killed for, had never loved her at all. It was a bitter pill to swallow.

Who killed Philip in the book?

There was more bad news for Kirsten when Dr Calgary made his belated appearance and announced, with much fanfare, that he was poor innocent Jack's alibi. The revelation that Jack couldn't have killed Rachel meant that another member of the household had done the terrible deed – and suddenly everyone was suspicious of everyone else.

No one knew anything much except for Tina, who had driven over and walked towards the house on the night in question. She'd overheard two people (later revealed to be Kirsten and Jack) making a whispered plan, but didn't know exactly who. At one point she wondered if it was her dad Leo and his secretary and soon-to-be fiancé Gwenda, but she couldn't be sure.

Kirsten panicked. When Mary's polio-stricken husband Philip started poking around and asking too many questions, she tried to warn him off – but when he wouldn't listen, she thought it was only fair to stab him in the back of the neck. And when Tina came running in and spotted damning evidence, Kirsten stabbed her too. (Don't worry: Tina survived. Just.)

With two murders and one attempted murder under her belt, Kirsten very cleverly framed Mickey. And she would have got away with it - if it weren't for that meddling Calgary.

In the end it was Dr Calgary the venerable physicist who gathered the family together to reveal who was guilty. He'd cracked the case by delving into Jack's romantic history and discovered his habit of seducing and fleecing middle-aged women, which threw Kirsten's actions into a whole new light.

The murderer fled, but – it is assumed – didn't get far.

Did Tina and Mickey get together in the book?

In the TV adaptation everyone freaked out when they discovered Tina and Mickey's secret relationship. Sure, they might not be biologically related – but they'd been raised as siblings! So weird!

This storyline originates from the book, although the idea that they're in love is only raised in the last couple of pages and we never see them actually get together.

But everyone seems strangely chilled out about it. When Calgary asks if Tina has the hots for her adoptive brother, Hester says: "I suppose she might be. I never thought about it. They've always been brother and sister, of course. But they're not really brother and sister."

She adds: "Perhaps they'll get married, when Tina gets well."

Is that... even legal?

Did Hester get drugged and tricked into an abortion in the book?

No. But looking at Christie's version of Hester, you can see where this storyline comes from.

When we meet her in the novel, flighty and fragile Hester's 'rebellion' is in the past, when she ran away to become a terrible actress and had an affair with a middle aged married man. She wasn't particularly keen on either the acting or the lover, but she was very keen on spiting her mother.

Rachel, who was actually a really attentive mother with a tendency to smother her kids with love, stepped in to save the day, breaking up the affair and offering to send her to a proper drama school. Her kindly interference left Hester furious, because her mother was always so annoyingly right about everything.

Was Jack the biological son of Kirsten and Leo?

No, absolutely not. This is a real twist in the TV adaptation.

Phelps' story makes Kirsten into Jack's mother, not his lover. She conceived Jack with her boss Leo when she was only 15, and stayed on in the household to watch him grow up as Rachel and Leo's "adpoted" son, thinking he'd been abandoned by his parents.

But in the novel, Jack was truly an abandoned and unwanted kid just like the rest.

Did the police try to cover things up?

In the TV adaptation, Calgary declines to get the police involved – firstly because he has only just been discharged from the asylum, and secondly because when he does ring them, paedophilic police boss Bellamy tries to run him over in a car and kills himself in the process. That's enough to put anyone off.

But in the book, the police were involved – they were just pretty ineffectual compared to amateur detectives Philip and Calgary. Superintendent Huish did his best to get to the bottom of the case, uncovering new evidence and establishing that Tina drove to the house after all. But it was Calgary who put it all together and cracked the case.

Was Arthur Calgary a psychiatric patient in the book?

No. Dr Arthur Calgary's account of the night was a lie in the TV version, but true in the novel.

In Christie's version of events, he was a physicist who had been on his way back to the train station after visiting friends in the area. He gave a lift to hitchhiker Jack, but shortly afterwards was hit by a car and suffered a terrible concussion, inconveniently losing his memory of the evening. As soon as he recovered he went off on an Arctic research expedition and didn't hear about the whole murderous affair until he spotted an old newspaper on his return and the memories came flooding back.

So book version Calgary didn't have to be rescued from an asylum and never suffered a meltdown about his contribution to nuclear destruction. Instead, he was the one to uncover the truth, and the reward for his labours was the love of Hester. "I think I'd like to marry you," she declared. And then they lived happily ever after. (Probably.)

This article was originally published in April 2018