Three Act Tragedy - Agatha Christie
A dinner party is thrown by theatre actor Sir Charles Cartwright at his home in Cornwall. His guests include Dr. Bartholomew Strange, Lady Mary Lytton Gore and her daughter Hermione "Egg", Captain Dacres and his wife Cynthia, Muriel Wills, Oliver Manders, Mr Satterthwaite, and the Reverend and Mrs Babbington. Cartwright mixes cocktails, which are passed to guests by a serving girl with a tray. After sipping one of the cocktails, Reverend Babbington collapses and dies. Cartwright is convinced it was murder. Investigation of the glass shows no poison, and the death is ruled natural causes at the inquest. Cartwright is so upset that he informs Poirot he intends to retire to Monte Carlo.
In his home in Yorkshire, Dr Strange hosts a party with many of the same guests at the party in Cornwall, though missing Sir Charles, Mr. Satterthwaite, and Poirot. Oliver Manders arrives in an unusual way, as his motorcycle breaks down in front of the manor. Dr Strange has a new, temporary butler named Ellis, who reports that a new patient has arrived at the sanatorium, a Mrs De Rushbridger, called "Mrs D". After dinner, Ellis serves port to all the guests, after which Dr Strange dies. Although no poison is found in his glass, the coroner determines that he died of nicotine poisoning. Given the similarities, Babbington's body is exhumed, revealing that he too died of nicotine poisoning.
Poirot learns of Dr Strange's death from Mr. Satterthwaite and Sir Charles in Monte Carlo. Satterthwaite and Sir Charles return to England, where they investigate the deaths. The serving maid at Dr Strange's manor notes that Dr Strange gave his usual butler a two-month vacation about two weeks before his death and brought on Ellis, now disappeared from the house. She said Ellis seemed to know a butler's duties, although he went about them in a strange way. In Ellis' room with Satterthwaite, Cartwright finds hidden papers suggesting that Ellis was blackmailing Dr Strange. In London, Muriel Wills reports that she noticed something unusual at the Yorkshire dinner. She observed that Ellis had a birthmark on his right hand. Poirot arrives and offers to counsel Sir Charles, Egg, and Mr. Satterthwaite on the investigation. Poirot then receives a telegram from "Mrs D" at Dr Strange's sanatorium. When Poirot and Satterthwaite arrives in Yorkshire, they find "Mrs D" murdered by nicotine poison hidden in chocolates sent to her anonymously. Poirot soon stops Miss Milray before she can destroy chemical equipment hidden in an abandoned building near the vicarage in Cornwall.
Poirot assembles everyone for the denouement. He reveals that Sir Charles Cartwright murdered Rev Babbington, Dr Strange, and "Mrs D". Sir Charles had wanted to marry Egg, but could not do so because he had a wife whom he had married many years ago, who is now in an insane asylum. Under British law, Cartwright could not divorce his wife. Cartwright would not have killed his wife in the asylum. But he would and did murder the only person who knew about her, his childhood friend Dr Strange. (In the American version, this is changed with Sir Charles' new motive being to prevent Dr Strange from committing him to an asylum for his growing megalomania.)
Cartwright used his party in Cornwall as a dress rehearsal for the murder of Strange. His wanted to test his success at switching the glass with the poison, with a glass untouched. He ensured that Dr Strange would not drink the poison at the first party because he knew his old friend did not drink cocktails. He ensured Egg did not drink the poison by handing her a glass. The rest of the cocktails were put on a tray to be distributed. It did not matter to Cartwright who died (the lack of any motive to kill Babbington had confused Poirot). Sir Charles then convinced Strange to let him play the role of the butler, Ellis.
Sir Charles was in Monte Carlo the day after Strange was killed, which he hoped would be his alibi. He had left the blackmail note from Ellis while still pretending to be Ellis. When Muriel Wills spoke up, Sir Charles was prepared to kill her, too. Poirot saw the risk and hid her. Sir Charles killed "Mrs D" because otherwise she would have told Poirot that she had not sent that telegram and that she was completely unconnected to the crime. Miss Milray was secretly in love with her long-time employer, and thus ready to destroy the equipment Sir Charles used to distill the nicotine poison. Cartwright is arrested and Egg winds up with Oliver Manders.