Poirot receives a visit from Miss Mary Marvell, the famous American film star on her visit to London. She has received three letters, handed to her by a Chinese man, which warn her to return her fabulous diamond jewel, the "Western Star", to where it came from– the left eye of an idol – before the next full moon. Her husband, Gregory Rolf, who bought it from a Chinese man in San Francisco, gave Mary the jewel three years ago. The pair are going to stay at Yardly Chase, the home of Lord and Lady Yardly when the moon is next full to discuss the makingof a film there and Mary is determined to go with her diamond. Both Poirot and Hastings remember society gossip from three years back that linked Rolf and Lady Yardly. The Yardlys also own an identical diamond that came from the right eye of the idol – the Star of the East. After Mary has gone Poirot goes out and Hastings receives a visit from Lady Yardly, who was advised to visit Poirot by her friend Mary Cavendish. Hastings deduces that she too has received warning letters. Her husband plans to sell their jewel as he is in debt. When Poirot learns this he arranges to visit Yardly Chase andis there when the lights go out and Lady Yardly is attacked by a Chinese man and her jewel stolen. The next day, Mary's jewel is stolen from her London hotel. Poirot makes his investigations and returns the Yardly's jewel to them.
Poirot tells Hastings that there never were two jewels or any Chinese man– it was all an invention by Rolf. Three years before in the USA he had an affair with Lady Yardly and blackmailed her into giving him the diamond which he then gave to his wife as a wedding present. Lady Yardly's was a paste copy that would have been discovered when her husband sold it. She was starting to fight back against her blackmailer and Rolf arranged the deception against his wife that Lady Yardly copied when Hastings told her of the threats. Poirot's threats manage to persuade Rolf to give the real diamond back and leave the Yardlys in peace.
Poirot is asked by a friend, who is the director of the Northern Union Insurance Company, to investigate the case of a middle-aged man who died of an internal haemorrhage just a few weeks after insuring his life for fifty thousand pounds. There were rumours that Mr Maltravers was in a difficult financial position and the suggestion has been made that he paid the insurance premiums and then committed suicide for the benefit of his beautiful young wife. Poirot and Hastings travel to Marsdon Manor in Essex where the dead man was found in the grounds, with a small rook rifle by his side. They interview the widow and can find nothing wrong. They are leaving when a young man, Captain Black, arrives. A gardener tells Poirot that he visited the house the day before the death. Poirot interviews Black and by using word association finds out that he knew of someone who committed suicide with a rook rifle in East Africa when he was there.
Poirot figures out that this story, told at the dinner table the day before the tragedy, gave Mrs Maltravers the idea of how to kill her husband by making him demonstrate to her how the African farmer would have put the gun in his mouth. She then pulled the trigger. Mr Maltravers is seen by a maid in the garden. She thinks that it is just a mistake, but then in the living room a strange thing happens. The lights suddenly go out and Mrs Maltravers clasps Poirot's hand. Mr Maltravers appears in the room, his index finger glowing and pointing at Mrs Maltravers' hand, which is covered in his blood. Terrified, she confesses. Poirot explains that he hired a man to impersonate Mr Maltravers and turn off the lights. When Mrs Maltravers grabbed Poirot's hand, he put fake blood on hers. The man applied phosphorescent to his finger to make it glow and pointed to the woman's hand, which was covered in fake blood.
Hastings is at a friend's house with several other people when the talk turns to flats and houses. Mrs Robinson tells the party how she and her husband have managed to obtain a flat in Knightsbridge for a very attractive price. Poirot is interested in this strange event and starts to investigate. The porter at the flat tells them that the Robinsons have been there for six months, despite Mrs Robinson telling Hastings they had only just obtained the lease. Poirot rents another flat in the building and, by use of the coal lift, manages to gain entry to the Robinson's flat and fix the locks so he can enter at will. Japp tells Poirot that important American naval plans were stolen from that country by an Italian called by Luigi Valdarno who managed to pass them onto an international spy, Elsa Hardt, before being killed in New York. Hardt's description is a close match to Mrs Robinson.
That night, when the Robinson's flat is empty, Poirot and Hastings lie in wait and apprehend another Italian who has come to kill Elsa Hardt and her accomplice in revenge for the death of Valdarno. They disarm the man and take him to another house in London. Poirot tracked down the two spies there, having previously lived in Knightsbridge as a Mr and Mrs Robinson. In fear of their lives, the spies rented the flat cheaply to a real couple of the same name, in hopes that this innocent couple would be killed in their place. They trick Hardt into revealing the hiding place of the plans before the Italian escapes and Japp arrests the two spies.
Poirot and Hastings receive a visit from a Mr Roger Havering, the second son of a Baronet who is married to an actress. Mr Havering stayed at his club in London the previous evening and the following morning received a telegram from his wife saying that his Uncle Harrington Pace was murdered the previous evening and to come at once with a detective. As Poirot is indisposed, Hastings sets off with Havering for the scene of the crime. Mr Pace, the brother of Mr Havering's mother, owns a hunting lodge on the Derbyshire moors. When Hastings and Havering arrive there they meet Inspector Japp as Scotland Yard has been called. As Havering goes off with Japp, Hastings speaks with the housekeeper, Mrs Middleton, who tells him she showed a black-bearded man into the house the previous evening who wanted to see Mr Pace. She and Mrs Zoe Havering were outside the room where the two men were talking when they heard a shot. The door to the room was locked but they found an open window; gaining entry, they found Mr Pace dead, shot by one of two pistols on display in the room. The pistol and the man are now missing. Mrs Middleton sends Zoe Havering to see Hastings and she confirms the housekeeper's story. Japp confirms Havering's alibi for his train times to London and his attendance at the club but soon the missing pistol is found dumped in Ealing. Hastings wires to Poirot with the facts but Poirot is interested only in the clothes worn by and descriptions of Mrs Middleton and Mrs Havering. Poirot wires back to arrest Mrs Middleton at once but she disappears before this can happen. Upon investigation, no trace can be found of her existence, either from the agency that sent her or how she reached Derbyshire.
Once Hastings is back in London, Poirot gives Hastings his theory– Mrs Middleton never existed. She was Zoe Havering in disguise. Only Mr Havering claims to have seen the two women together at the same time. Havering did go to London with one of the pistols which he dumped and Mrs Havering shot her uncle with the other pistol. Japp is convinced of the theory but does not have enough evidence to make an arrest. The Haverings inherit their uncle's fortune but not for long: the two are soon killed in an aeroplane crash.
The fiancée of Philip Ridgeway asks Poirot to prove his innocence. Ridgeway is the nephew of Mr Vavasour, the joint general manager of the London and Scottish Bank and a million dollars of bonds have gone missing whilst in his care. Poirot meets Ridgeway at the Cheshire Cheese to hear the facts of the case:Ridgeway was entrusted by his uncle and the other general manager, Mr Shaw, with taking a million dollars of Liberty bonds to New York to extend the bank's credit line there. The bonds were counted in Ridgeway's presence in London, sealed in a packet and then put in his portmanteau that had a special lock on it. The packet disappeared just a few hours before the liner on which Ridgeway was travelling, the Olympia , docked in New York. Attempts had been made to break into the portmanteau but its lock must have then have been picked. Customs were alerted and they sealed the ship that they then searched but to no avail. The thief was selling the bonds in New York so quickly that one dealer swears to buying some bonds before the ship docked. Poirot then questions the two general managers who confirm this story. He then travels to Liverpool where the Olympia has just returned from another crossing and the stewards confirm the presence of an elderly man wearing glasses in the cabin next to Ridgeway, who never left it.
Poirot meets back with Ridgeway and his fiancée and explains the case to them. The real bonds were never in the portmanteau. Instead they were posted to New York on another faster liner, the Gigantic , which arrived before the Olympia . The confederate at the other end had instructions to begin selling the bonds only when the Olympia docked but he failed to carry out his orders properly, hence one sale took place half an hour before docking. In the portmanteau was a false packet that the real villain took out with a duplicate key and threw overboard– this was Mr Shaw who claims he was off work for two weeks due to bronchitis whilst these events transpired. Poirot caught him by asking him if he can smoke a cigar, which, given his bronchitis, Shaw should have immediately declined. However, he said yes, convincing Poirot of his guilt.
Lady Willard consults Poirot. She is the widow of the famous Egyptologist, Sir John Willard. He was the archaeologist on the excavation of the tomb of the Pharaoh Men-her-Ra together with an American financier, Mr Bleibner. Both men died within a fortnight of each other, Sir John of heart failure and Mr Bleibner of blood poisoning. A few days later Mr Bleibner's nephew, Rupert, shot himself and the press is full of stories of an Egyptian curse. Lady Willard's son, Guy, has now gone out to Egypt to continue his father's work and she fears that he will die next. To Hastings' surprise, Poirot states that he believes in the forces of superstition and agrees to investigate. Poirot cables New York for details concerning Rupert Bleibner. The young man was an itinerant in the south seas and borrowed enough money to take him to Egypt. His uncle refused to advance him a penny, and the nephew returned to New York, where he sank lower and lower and then shot himself, leaving a suicide note saying that he was a leper and an outcast.
Poirot and Hastings travel to Egypt and join the expedition, only to find that there has been another death in the party, that of an American by tetanus. As Poirot investigates the dig, he feels the forces of evil at work. One night, an Arab servant delivers Poirot his cup of chamomile tea. Hastings hears Poirot choking, after drinking the tea. He fetches the expedition surgeon, Dr Ames. This is, however, a pretext to get the doctor into their tent where Poirot orders Hastings to secure him. The doctor, however, quickly swallows a lethal cyanide capsule. Poirot explains that Rupert was Bleibner's heir, and the doctor, secretly, must have been Rupert's heir. Sir John died of natural causes. His death started superstitious speculation. Everyone assumed that Rupert's friend in the camp was his uncle but that could not have been the case as they argued so frequently. Despite having no money, Rupert returned to New York, which shows that he did have an ally in the expedition. This was a false ally– the doctor, who told Rupert he had contracted leprosy in the south seas and it must be part of the curse. (Rupert merely had a normal skin rash.) After Dr Ames killed his uncle, Rupert believed himself cursed and shot himself. His note refers to the leprosy, which everyone assumed was a metaphorical reference, not a real condition.
Poirot and Hastings are staying at the Grand Metropolitan hotel in Brighton where they meet Mr and Mrs Opalsen. He is a rich stockbroker who amassed a fortune in the oil boom and his wife collects jewellery using the proceeds. She offers to show Poirot her pearls and goes to fetch them from her room but they have been stolen. Poirot is asked to assist. There have only been two people in the room since the pearls were last seen - Mrs Opalsen's maid, Celestine, and the hotel chambermaid. Celestine has orders to remain in the room all of the time that the chambermaid is there. Both girls are questioned and each blames the other. The hotel room has a side room where Celestine sleeps and a bolted door which leads to the room next door. The two maids were in sight of each other all the time except for two pauses of between twelve and fifteen seconds apiece when Celestine went into her room– not enough time to extract the jewel case from the drawer, open it, take the jewels and return the case. Both are searched but nothing is found. Both rooms are then searched and the missing pearls are found underneath Celestine's mattress. The case is seemingly over but Poirot tells Hastings thenewly found necklace is a fake. He questions the chambermaid and the valet who looks after Mr Opalsen and asks them if they have ever seen before a small white card he has found. Neither has.
Poirot rushes to London and the next day breaks the news to Hastings and the delighted Opalsens that the case is solved and the real pearls found. The chambermaid and the valet were a pair of international jewel thieves– the card he gave them then had their fingerprints on it which he gave to Japp for testing. The valet was on the other side of the bolted door and the chambermaid passed him the case in the first interval when Celestine was in her room. When she next went in there, the chambermaid returned the empty case to the drawer whose runners had been silenced with French polish, traces of which Poirot found in the room next door. The pearls are found in the valet's room and returned to their grateful owners.
Towards the end of the First World War, Hastings calls on Poirot in his rooms to discuss the sensational news of the day - the attempted assassination of the Prime Minister, David MacAdam. They are interrupted by two visitors: Lord Estair, Leader of the House of Commons and Bernard Dodge, a member of the War Cabinet. They enlist Poirot to help with a national crisis– the Prime Minister has been kidnapped. He was on his way to a secret peace conference to be held the next day at Versailles. He arrived in Boulogne-sur-Mer where he was met by what was thought to be his official car but it was a substitute. The real car was found in a side road with its driver bound and gagged. As they tell Poirot the details, news reaches them by special courier that the bogus car has been found abandoned and containing Captain Daniels, the Prime Minister's secretary, chloroformed and gagged. His employer is still missing. Poirot wants to know the full details of the shooting that took place earlier. It occurred on the way back from Windsor Castle when, accompanied by Daniels and the chauffeur, Murphy, the car took a side road and was surrounded by masked men. Murphy stopped and one man shot at the P.M., but only grazing his cheek. Murphy drove off, leaving the would-be murderers behind. The P.M. stopped off at a small cottage hospital to have his wound bandaged and then went straight on to Charing Cross Station to get the Dover train. Murphy has also disappeared, the P.M.'s car being found outside a Soho restaurant frequented by suspected German agents. As Poirot packs to leave for France he voices his suspicions of both Daniels and Murphy and wonders why the shooting by masked men took place before the kidnap. Poirot goes over the Channel with various detectives involved in the case, among them Japp. Once in Boulogne he refuses to join in the search but sits in his hotel room and thinks for several hours.
Abruptly, he returns to Britain where he begins a tour of cottage hospitals to the west of London in an official car. They call at a house in Hampstead; the police raid it and recover both Murphy and the Prime Minister. The villain was Daniels who kidnapped both men in the shooting, taking to London substitutes with the "P.M.'s" face disguised by bandages from a shooting that had never occurred; Poirot's search of the cottage hospitals proved that no one's face was bandaged up that day. Investigation was misdirected to France, when the real P.M. had never left the country. Daniels was known to have a sister near Hampstead but she is Frau Bertha Ebenthal, a German spy for whom Poirot has been searching for some time. The real Prime Minister is whisked off to Versailles for the conference.
Poirot and Hastings are entertaining Japp after they had all attended a magic show when the conversation turns to the recent disappearance of a banker, Mr. Davenheim, from his large country house, the Cedars. Boasting, Poirot makes a five-pound bet with Japp that he could solve the case within a week without moving from his chair. The facts of the case are that Davenheim arrived home from the city at midday on Saturday. He seemed normal and went out to post some letters late in the afternoon saying that he was expecting a business visitor, a Mr. Lowen, who should be shown into the study to wait his return. Mr. Davenheim never did return and no trace of him can be found. The police were called on Sunday morning and on the Monday it was discovered that the concealed safe in Davenheim's study was forced open and the contents removed– cash, a large amount of bearer bonds and jewellery. Lowen has not been arrested, but is under observation. He was there to discuss some business in South Africa with Mr. Davenheim, who was in Johannesburg the previous autumn. Poirot is interested that the house has a boating lake, which Japp tells him is being searched tomorrow, and that Mr. Davenheim wears his hair rather long with a moustache and bushy beard. The next day Japp returns with the news that Davenheim's clothes have been found in the lake and that they have arrested Lowen. A common thief called Billy Kellett, known to the police for pick-pocketing, saw Lowen throw Davenheim's ring into a roadside ditch on the Saturday. He picked it up and pawned it in London, got drunk on the proceeds, got arrested and is in custody.
Poirot has one question for Japp: Did Mr. and Mrs Davenheim share a bedroom? When the reply is returned in the negative, Poirot knows the solution and tells Hastings and Japp to withdraw any funds they have in Davenheim's bank before it collapses. This predicted event occurs the next day and Poirot reveals the truth. Davenheim knew of his bank's financial troubles and started to prepare a new life for himself. Last autumn he did not go to South Africa but instead took on the identity of Billy Kellett. He served three months in jail at the same time he was supposed to be in Johannesburg and then, to set up Lowen, on the Saturday robbed his own safe before Lowen arrived at the house. When Davenheim 'disappeared' he was already in police custody as Kellett and no one looked for a missing man in jail. Mrs Davenheim identifies her husband and Japp pays Poirot his five pounds.
Poirot and Hastings are in their rooms with a neighbour, Dr Hawker, when the medical man's housekeeper arrives with the message that a client, Count Foscatini, has phoned for the doctor, crying out for help. Poirot, Hastings and Hawker rush to Foscatini's flat in Regent's Court. The lift attendant there, unaware of any problems, says that Graves, the Count's man, left half an hour earlier with no indication of anything wrong. The flat is locked but the manager of the building opens it for them. Inside, they find a table set for three people, with the meals finished. The Count is alone and dead– his head crushed in by a small marble statue. Poirot is interested in the remains on the table. He questions the kitchen staff at the top of the building. They describe the meal they served and the dirty plates passed up to them in the service elevator. Poirot seems especially interested in thefact that little of the side dish and none of the dessert were eaten, while the main course was consumed entirely. He also points out that after crying out for help on the phone, the dying man replaced the receiver.
The police arrive at the flat as Graves returns. He tells them the two dinner guests first visited Foscatini on the previous day. One was a man in his forties, Count Ascanio, and a younger man. Graves said that he listened to their first conversation, and heard threats uttered; then the Count invited the two men to dinner the next evening. Graves says that the next night Foscatini unexpectedly gave him the night off after dinner, when the port had been served. Ascanio is quickly arrested, but Poirot states three points of interest: the coffee was very black, the side dish and dessert were relatively untouched, and the curtains were not drawn. The Italian ambassador provides an alibi for Ascanio, which leads to suspicions of a diplomatic cover-up, and Ascanio denies knowing Foscatini. Poirot invites Ascanio for a talk and forces him to admit that he did know that Foscatini was a blackmailer. Ascanio's morning appointment was to pay him the money he demanded from a person in Italy, the transaction being arranged through the embassy where Ascanio worked.
After Ascanio leaves, Poirot tells Hastings that Graves is the killer and explains his reasoning. Graves overheard the monetary transaction, and realised that Ascanio could not admit to the relationship with Foscatini. Graves killed Foscatini when he was alone– there never were any dinner guests - then ordered dinner for three and ate as much of the food as he could; but after consuming the three main courses, he could eat only a little of the side dishes and none of the desserts. Coffee was served for three and presumably drunk, but Foscatini's brilliant white teeth show that he never drank such staining substances. Finally, the open curtains show that Graves left the flat before nightfall and not after, which would not have been the case if the account given by Graves were true. This theory is passed on to Japp, and when he investigates, Poirotis proven right.
Poirot receives an unusual request from Miss Violet Marsh. She was orphaned at fourteen years of age, when she went to live with her Uncle Andrew in Devon. He was recently returned from making his fortune in Australia. He was opposed to his niece bettering herself through book learning. Violet rebelled against him and got herself into Girton College some nine years earlier. Although somewhat strained, her relations with Andrew Marsh remained cordial. He died a month earlier, leaving a will with a strange clause. The will is dated 25 March and timed at 11:00 am. Marsh gave instructions that his "clever" niece was allowed to live in his house for one month; in that time she had to "prove her wits". If at the end of that time she had not done so, all his worldly goods would go to charitable institutions and she would be left with nothing.
Poirot and Miss Marsh are convinced that there is either a second will or a sum of money hidden in the house; he agrees to look. Poirot and Hastings travel to Devon, where they are looked after by Mr and Mrs Baker, Marsh's housekeepers. They tell Poirot that they signed and witnessed two wills, as Marsh said he had made a mistake with the first, although they did not see its contents. Immediately afterwards, Marsh left the house to settle tradesmen's accounts.
Looking round the house, Poirot is pleased with the dead man's order and method, except for one particular: the key to a roll top desk has been affixed, not to a neat label, but instead to a dirty envelope. After a long search, Poirot declares himself beaten and is about to return to London when he remembers the visit to the tradesmen. He rushes back to the house and holds up the opened envelope to the fire. Faint writing in invisible ink appears, which is a will dated after the one in Violet's possession and leaving everything to her. The two wills signed by the Bakers were a ruse. The tradesmen in town must have signed the true last will, which Marsh turned into an envelope attached to a key. This was left as a deliberate clue. According to Poirot, Miss Marsh proved her superior intelligence by employing Poirot to solve the case.