The story is very clever. But, I've always wondered how the killer was able to plan the murder centered around the dentist. How did the killer know Amberiotis was going to that dentist on that day and time?
The novel does not say.
The day at the dentist’s would have required planning about the shooting, the injection and Blunt being seen to leave. But this would have been simple compared with organising the appointment structure. Disappointingly, however, we are not told how Amberiotis became a new patient of Blunt’s dentist, how Blunt found out about this or about the timing of the appointment or how he secured the immediately preceding appointment while allowing enough time for the ‘false’ Miss Sainsbury Seale to be fitted in later.
John Goddard (2018). Agatha Christie’s Golden Age, chapter 19. Stylish Eye Press.
However, perhaps we can imagine a suitable sequence of events. What we can reliably deduce from the novel is that:
- Miss Sainsbury Seale “came home from India on the same boat as Amberiotis” where they met.
- “About three months ago” Miss Sainsbury Seale recognized Mr Blunt in the street and told him “I was a great friend of your wife's, you know”.
- “About a week or so before he died”, Mr Amberiotis had lunch at the Savoy with Miss Sainsbury Seale, where she told him that her friend Gerda had married Mr Blunt.
- Mr Amberiotis deduced that Mr Blunt was a bigamist and phoned or wrote to blackmail him.
- “Gerda went to see her [Miss Sainsbury Seale], asked her to tea … Mabelle Sainsbury Seale came, quite unsuspecting. She never knew anything—the medinal was in the tea.”
- Gerda, impersonating Miss Sainsbury Seale, paid the latter’s bill at her “hotel near Russell Square” and took her belongings to the Glengowrie Court Hotel, where she continued to impersonate her.
- Mr Amberiotis booked an appointment with Mr Morley (the dentist). In chapter 2 his notebook already contains the time and place, “Twelve o'clock. 58 Queen Charlotte Street.”
- Gerda, impersonating Miss Sainsbury Seale, rang Mr Morley and managed to persuade him to fit her in on the same day as Mr Amberiotis. We get Mr Morley’s account of the call in chapter 1: “A full morning, and that Sainsbury Seale woman to fit in somewhere as she is in pain. I suggested that she should go to Reilly, but she wouldn't hear of it.”
- Mr Blunt murdered Mr Morley and Mr Amberiotis with Gerda’s assistance.
So we have to imagine some additional events. One possible schedule is as follows:
- (a) When Mr Amberiotis was having lunch at the Savoy with Miss Sainsbury Seale, he told her about his toothache.
- (a) While Miss Sainsbury Seale was drinking the poisoned tea at Gerda’s flat, she chattered in her usual way and mentioned that Mr Amberiotis was suffering from toothache.
- (a) Gerda, impersonating Miss Sainsbury Seale, contacted Mr Amberiotis and recommended Mr Morley to him.
- (a) Gerda discovered the date and time of Mr Amberiotis’ appointment by ringing up (as Mrs Chapman, say) for an appointment, mentioning “her friend Mr Amberiotis” to the receptionist (Gladys Nevill), and worming the time (12:00) out of her. We know that Gladys was fooled by the false telegram, so she may have been easy for Gerda to manipulate.
- (b) Mr Blunt (or Gerda acting as his secretary) rang up and insisted on getting the 11:30 appointment. We know that Mr Blunt was an important man financially and politically, so he may have been able to bully the receptionist into moving another patient, or he may have gotten lucky and the slot was available.
These points may or may not work as there will be too much going on around the appointment's time and Mr Morley's secretary would become a bit suspicious or at least would notice. Nothing of the above points are mentioned in the novel hence it's a whole lot of deduction. What is mentioned though in the first chapter is that Mr Amberiotis is using a toothpick while happily thinking about some prospect (getting blackmail money from Blunt), he gets a bit careless and pricks himself hence the sudden visit to the dentist that day. By that time Miss Sainsbury was already dead. There is no way that Mr Blunt and his wife could be aware of when and why Amberiotis was going to visit that same dentist. They planned the dentist's murder so he can't identify Sainsbury and also to switch the dental records so it would seem that Mrs Chapman had died. So there can be no connection between Amberiotis and Sainsbury regarding a dental issue at that time (Sainsbury was already dead and the dentist's murder was planned for that day) that could have coincidentally and randomly come to the attention of The Blunts making them adjust the murder of Amberiotis in the same dental visit. That is a huge plot hole. Also the fact that Amberiotis chose the same dentist for his first visit is a bit odd but may be he was the only famous and good dentist in the area of his hotel (nearby to where Blunt, his wife and the deceased Sainsbury lived). Plus the Blunts couldn't have thought of it that day to kill Amberiotis too by just spotting him at the dentist as their appointment times was carefully planned and they used the dentist's death as an excuse to cover up for Amberiotis murder. And also used his anesthetics to kill him. Both murders were so carefully preplanned but the key information of how would they know that Amberiotis will be there at that certain time is so grossly overlooked. It is extremely annoying.