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We've taken a deep dive into the final episode, with a closer look at the ending and what exactly has been changed from the novel – aside, of course, from the controversial disappearance of Poirot's companion Arthur Hastings and the untimely death of Inspector Japp.

(And in case it needs to be said: SPOILER WARNING!)

Was the ending changed from the novel?

If you were expecting the BBC's go-to Agatha Christie screenwriter Sarah Phelps to change the ending AGAIN, surprise! She's defied expectations... and kept the murderer exactly the same.

The ABC killer is none other than Franklin Clarke.

The solution to the crime remains true to Christie's original ending. So what was he playing at?

Franklin invented an anonymous homicidal murderer to disguise his true objective: murdering his brother and inheriting the money. He sent sinister letters to Poirot, but as the detective remarks in the novel, "They were a fake! They pretended to be the letters of a madman - of a homicidal lunatic, but in reality they were nothing of the kind." He explains: "It was to focus attention on several murders – on a group of murders... when do you notice a pin least? When it is in a pin cushion! When do you notice an individual murder least? When it is one of a series of related murders."

While he was considering his options, Franklin bumped into a man called Alexander Bonaparte Cust and came up with a clever scheme to frame him for a series of "alphabetical" murders – including the murder of Carmichael Clarke. Cust was extremely suggestible and clearly very ill, and would be the perfect "stalking horse."

Franklin wrote to Cust pretending to be a firm selling stockings, and offering him a job as a salesman. He typed out the ABC letters and the gave him the incriminating typewriter, and he had a large consignment of stockings pre-delivered to his new boarding house (as well as a secret parcel of ABC railway guides). Crucially, he also sent Cust a list of instructions and places to visit on behalf of the firm: Andover, Bexhill, Churston and Doncaster.

Franklin bought Cust a Poirot-style outfit (apparently on behalf of the firm) and, once Cust was ready to go, it was time to start sending Poirot the ABC letters with the words "IT BEGINS."

What are the differences between the TV show and the book?

1. Franklin Clarke and his Poirot obsession

TV drama: In this version of the story, Franklin Clarke (Andrew Buchan) is utterly obsessed with Hercule Poirot – and although his initial motive for the killings is to get rid of his brother and inherit his wealth, he has become increasingly engrossed in playing this game with the Belgian detective. Over his last meal before his execution, he reveals that he was inspired by the "murder party" Poirot hosted at Sir Carmichael's house five years beforehand, and devised the scheme of the ABC murders because he wanted to be a worthy adversary. That's why he played mind games with Poirot by linking all the crimes to events from his past.

Novel: Although Poirot puzzles over the question of why the ABC killer is writing to him rather than Scotland Yard or a newspaper, the reason turns out to be mainly practical: Franklin deliberately sent the "Churston letter" to the slightly wrong address so that it would be delayed in the post, a plan which required a residential address like Poirot's. That delay meant Poirot could not warn Carmichael Clarke before it was too late. "The letters were sent to me because the essence of your plan was that one of them should be wrongly addressed and go astray," Poirot explains. Other than the letters being delivered to his house, none of the murders have any connection to Poirot's past.

2. Inspector Crome v. Hercule Poirot

TV drama: Inspector Crome (Rupert Grint) and Hercule Poirot (John Malkovich) go head-to-head in the TV drama, with young Crome going on the attack – refusing to listen to Poirot, insulting him mercilessly and confiscating the contents of his flat.

His predecessor at Scotland Yard was Inspector Japp, Poirot's long-time friend and collaborator, but Japp has recently retired and suffered a fatal heart attack. Now, Crome is determined to have nothing to do with Poirot. However, he comes to respect Poirot and realise that he needs his help.

Novel: Inspector Japp is a lot more fortunate in the novel and works alongside Poirot on the case. But after the Bexhill murder, our Belgian detective does have to content with an up-and-coming inspector called Crome. The book's narrator, Hastings, explains: "Crome was a very different type of officer from Japp. A much younger man, he was the silent, superior type. Well educated and well read, he was, for my taste, several shades too pleased with himself." Although he has a good track record "his manner to Poirot was a shade patronising."

But espite their quiet animosity, Poirot and Crome never descend into outright hostility.

3. What does Thora Grey know?

TV drama: Sir Carmichael Clarke's conniving secretary (Freya Mavor) is hoping to marry him once his wife Lady Hermione finally dies, but his rejection of her romantic advances and then his brutal murder puts an end to that. Thora knows that Franklin was the killer after he rushes into the bathroom covered in blood, but decides to throw her lot in with him and hope to become HIS wife when he inherits the family fortune. Franklin is, in fact, planning to propose.

Novel: Thora Grey IS hoping to marry Sir Carmichael as soon as he's a widower, much to Lady Hermione's horror. And after Sir Carmichael's death, she transfers her affections to his brother. But in the novel, there's no suggestion that she knows anything about Franklin being the killer.

4. The racism – and Poirot's past

TV drama: Racism and anti-immigrant feeling in the Britain of 1933 is a central theme of this Agatha Christie adaptation, as the public mood shifts against foreigners. The rise of the British Union of Fascists and the facts of the ABC case force Poirot to look back at his own past, when he fled Belgium in 1914: it is revealed that he was a Catholic priest who encouraged his congregants to shelter in his church and then saw the building (and its inhabitants) torched to the ground.

Novel: This dramatic storyline about Poirot's past does NOT come from the novel, although anti-foreigner feelings are present in the original story. Poirot detects a "a slight anti-foreign bias" in the first ABC letter, which reads: "You fancy yourself, don't you, at solving mysteries that are too difficult for our poor thick-headed British police?" And when Franklin is identified as the killer, he yells: "You unutterable little jackanapes of a foreigner." Which is a brilliant line.

5. Ernie Edwards in Embsay

TV drama: After the botched murder in Doncaster, ABC's final victim is Ernie Edwards in Embsay, at which point Franklin is able to plant the murder weapon on Cust while he's having a seizure in the train station toilet.

He has gained a taste for murder, and he plans to keep working through the alphabet until the plan has worked – and Cust is behind bars. He also admits he may not have been able to stop at all.

Novel: The murders stop at D. Franklin had been hoping that Cust would be arrested by the police after C, but he's apparently so forgettable that Thora doesn't even recall talking to the stocking salesman at the door on the day of Carmichael's murder.

Franklin then scrambles to arrange a murder in Doncaster, but he doesn't enjoy murder for murder's sake. Poirot explains: "After the death of your brother, of course, your object was accomplished. You had no wish to commit any more murders. On the other hand, if the murders stopped without reason, a suspicion of the truth might come to someone."

6. Fingerprints on the typewriter

TV drama: Poirot matches the "unknown fingerprints" on Cust's typewriter with Franklin's prints, as collected from his brandy glass. This is used as part of his conviction.

Novel: Poirot tells Franklin: "Most damning of all – you over-looked a most elementary precaution. You left a fingerprint on Cust's typewriter – the typewriter that, if you are innocent, you could never have handled." Franklin immediately admits to the murders, but Poirot later admits to his friend Hastings that he'd made that up entirely in order to elicit the full confession.

7. Franklin tries to shoot himself

TV drama: Franklin is arrested by Crome, tried, and sentenced to death. He has a final one-on-one interview with Poirot and then goes off to his execution.

Novel: After his crimes are discovered, Franklin tries to kill himself, saying: "You win, M Poirot! But it was worth trying!" and whipping out a small automatic from his pocket, holding it to his head and firing. There is no response. Poirot's servant has pick-pocketed him and removed the bullet. Poirot tells him: "No, Mr Clarke, no easy death for you."

8. Cust's affair with Lily Marbury

TV drama: Landlady Mrs Rose Marbury pimps out her daughter Lily Marbury (Anya Chalotra) for sex and whatever her gentleman lodgers may desire, but Lily and Cust start to fall for each other romantically. When the police arrive to arrest Cust, Lily helps him make a getaway and later waits by his bedside after his brain surgery.

Novel: The boarding house is much less sleazy in the original novel, and the landlady's daughter Lily is pimped for sex; she lives a 'respectable' life and has a boyfriend. But she does feel sorry for Cust, especially after her boyfriend tells the police that he could be ABC, and calls him on the phone to warn that the police are on the way. After his innocence is proven, Cust tells Poirot: "I want to give a nice wedding present to Lily Marbury – a dear girl."

9. Donald Fraser and Megan Barnard

TV drama: Nasty Donald Fraser was dating Megan Barnard, but dumped her for her prettier sister Betty Barnard – who was actually only interested in his pay packet and was busy seeing other men and lying to his face. Megan was deeply hurt, and Donald rubbed salt into the wound by telling her she should have made more of an effort to impress him.

After Betty's death, Megan continues running after Donald and making excuses for him. But when he decides to marry her after all, she realises what a horrible man he is and makes her getaway. Hurrah!

Novel: Donald Fraser was infatuated with Betty and was upset about her involvement with other men – but still insisted they were in love. However, after her death he begins to fall for her sister Megan (who has always had a secret crush on him). With Poirot's encouragement, he declares his love.

10. Cust's alibi

TV drama: Lily Marbury admits – just to Poirot – that Cust does have an alibi for one of the murders, because he had a "booking" with her shortly afterwards and could not have made it home in time.

Novel: In the original story, Cust is actually NOT present for all the murders, because – to Franklin's dismay – he has an alibi for the night of the Bexhill murder. The police are still sure he's the killer, but Poirot has his doubts.

11. Cust's seizures and brain growth

TV drama: Cust has a growth on his brain which is causing these increasingly-dangerous seizures and absences. Doctors operate to remove the growth and Lily sits by his bedside.

Novel: Cust suffers from it's epilepsy and extreme headaches, although Poriot has an easy solution for the latter: "What about a visit to an oculist. Those headaches, it is probably that you want new glasses..."

And while he doesn't finish the story with a love interest, Cust is delighted to sell his story to the newspapers for several hundred pounds.

This article was originally published in December 2018

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