The ABC Murders BBC episode 1 recap and questions: Who is the killer? What is Poirot's past? BBC1, ABC in Australia

Adapted as a three-part drama by the BBC's go-to Agatha Christie screenwriter Sarah Phelps, The ABC Murders remains true to the plot of the novel while taking us in some dramatic new directions.

Here are the three big questions we're asking after that menacing first episode...

1. Who is Mr Cust – and is he really the murderer?

From his first moments on screen when he takes up a tenancy at Mrs Marbury's seedy London boarding-house, Alexander Bonapart Cust (A.B.C.) seems to be our serial killer, and every development in the story appears to prove his guilt.

As a travelling salesman, Cust (played by Eamon Farren) carries a suitcase full of boxes of Twinkle Toes stockings, along with an ABC railway guide. The unsmiling "Mr Cust" looks sickly and haunted, but he has high hopes for his "new enterprise," as he tells landlady Mrs Rose Marbury (Shirley Henderson). One of the only personal possessions in his rented room is his typewriter, perhaps used to write those sinister warning letters to Hercule Poirot.

On 31st March, Cust visits old Alice Asher in Andover and tries to sell her his stockings, although she tries to turn him away. Shortly afterwards, she is found dead in the back room of her shop. A blood-spattered railway guide is open next to "A" next to her body – and she is wearing a pair of stockings.

Cust then travels on to Bexhill, where he encounters the awful Betty Barnard. She takes a pair of his stockings – but turns sour when he asks her to pay for them. Instead, she calls him a virgin and taunts him with a string of verbal abuse that leaves him humiliated.

The next day, on 4th April – the date specified in the ABC letter to Poirot – Betty is found strangled with a stocking. The railway guide is open to "B".

Back in London, Cust regains consciousness and discovers cuts on his face and a large quantity of blood.

At this point, the evidence points so clearly towards Cust that surely he cannot be the murderer? Agatha Christie wouldn't ruin the suspense by revealing her killer so early in the story and in such an obvious way.

But if Cust isn't our serial killer, then how do you account for all the events described above? Or is Christie double-bluffing us?

2. Is Hercule Poirot keeping big secrets about his past?

"Your word is worthless. You're not who you say you are," Inspector Crome (Rupert Grint) accuses Hercule Poirot. "Who the hell are you?"

The moustachioed Belgian detective's personal history is not really explored in the novels, but for her latest Agatha Christie adaptation, screenwriter Sarah Phelps has made Poirot himself into the central mystery of the drama. Who is Poirot and what secrets does he hold?

Poirot's old pal at Scotland Yard, Inspector Japp, is no longer in the picture, having quietly retired and then died of a heart attack on his allotment. Instead, Inspector Crome is in charge, and he is determined to unmask the "truth" about this foreigner. The year is 1933 and the public mood has shifted; the British Union of Fascists is on the rise and its flash-and-circle badge has started to appear on people's lapels and on anti-immigrant posters.

According to the police's investigations, Poirot's backstory is fishy. "No one had ever heard your name. No Hercule Poirot registered anywhere in the Belgian police before 1914," Crome informs him. "Japp staked his reputation, his whole career, on you, and that was him over. His funeral should have been packed, a guard of honour, but all he got was a handful of old timers and you and me."

"You are lucky you never lived through an invasion," Poirot hits back. "Things are burnt. Buildings, people, records turn to ash."

But perhaps there is an element of truth to Crome's "findings," because John Malkovich's Poirot is clearly troubled by something that happened in Belgium during the First World War.

When he prays, and at other moments of reflection, he has painful flashbacks. Over and over, he recalls the face of a terrified young man, and remembers a French girl shouting "They're coming!" while scared civilians pray in French in a small church.

What happened to Poirot? And if he wasn't a policeman before the war, who was he?

3. What is the motive behind these killings?

Eve Austin as Betty Barnard in The ABC Murders

Looking at the murders individually, there are certainly some obvious suspects.

The police in Andover reckon it was local scoundrel Mr Asher who killed Mrs Asher, after previously making violent threats towards her. Despite Poirot's protestations that Mr Asher would be too weak to kill his wife and too desperate for beer money to leave the cash register untouched, he has been left to languish in a jail cell for now.

As for Betty Barnard, she was a real piece of work. Her fiancé Donald Fraser (Jack Farthing) would have had a motive to kill her, having just found out about her betrayal; her sister Megan Barnard (Bronwyn James) also had an axe to grind after Betty "stole" Donald from her.

"Our special darling, our perfect angel," Megan tells Poirot. "My sister wasn't either of those things. She was a slut."

But if both deaths are the work of a serial killer called ABC, all those motives become redundant. The personal circumstances of the victims are secondary to ABC's mission to work his way through the alphabet.

Instead, we have to ask: why did this self-titled "faceless beast" select Alice Asher and Betty Barnard as his first victims? Were they randomly chosen from a phone book for their alliterative names? Or is there a deeper motive?

If Cust is our killer, this could be about revenge: Betty totally humiliated him when he asked her to pay for the stockings. But then again, by the time they met at the Ginger Cat tea rooms, the letter to Poirot must already have been written and posted – and the victim already selected. So that doesn't quite add up.

The ABC Murders is an unusual Agatha Christie story in that it throws the mystery wide open. Instead of a small cast of suspects who Poirot can gather together in one room, this killer is truly "faceless". Will Poirot be able to solve the mystery?

This article was originally published on 27 December 2018