R21 It’s not a “television version”. It’s a look at the crime (as detected by Poirot) as if it actually happened to see whether the solution is feasible.
It’s not because the timeline doesn’t make sense.
SPOILERS SPOILERS SPOILERS SPOILERS (Please, if you haven’t read The Murder of Roger Ackroyd amd think you might, you really, really don’t want to read on).
So, according to Poirot...
Dr Shepherd murdered RA at 8.50. He then had to be at the Gate Lodge at 9pm on the dot to give the impression that he was just arriving. So that gives him 10 minutes to cover up his involvement and implicate Ralph Patton.
In that time he had to put on RP’s hobnail boots (stolen the night before from RP’s hotel room), walk around the study outside, go back in and leave more fake footprints, lock the door from the inside, go back out, change back into his own shoes, head over to the hotel to replace the boots and then get to the Gate House.
10 minutes is not enough time for this even if he ran to the hotel and back. In the show it was timed out as around 22 minutes.
Carrying around the dictaphone & boots in his doctor’s bag all day with no one noticing is not feasible either. In 1927, dictaphones were the size of typewriters. It would have been a struggle to fit in into a standard doctor’s bag plus the hobnail boots (presumably along with all his doctor bits).
This is enough for reasonable doubt in a court room.
The alternative suspect is Flora, Shepherd’s sister. It’s mentioned that if she had a crest it would be a mongoose. In Kipling’s stories, the mongoose kills to protect it’s family. She doesn’t have an alibi & is missing from Poirot’s denouement. Everyone else, except the parlour maid, has an alibi.
Finally, they make the point that when you have an unreliable narrator, you invoke the Liar’s Paradox. If Shepherd has been lying (if only by omission) throughout his narrative then we have absolutely no good reason to believe him when he implies that he’s the killer in his final paragraphs. He could have been protecting Flora, who killed to protect him.
Personally, I think if you plotted out all of Agatha Christie’s plots as if they were real world events most would have similar problems, but it’s fun to consider them anyway.