Three Act Tragedy

Renowned stage actor Sir Charles Cartwright hosts a dinner party in Cornwall at his home. His guests include Hercule Poirot; psychiatrist Sir Bartholomew Strange; Hermione "Egg" Lytton Gore and her mother; Captain Dacres and his wife Cynthia; the playwright Muriel Wills; Egg's friend Oliver Manders; Mr Satterthwaite; and Reverend Babbington and his wife. When the Reverend Babbington suddenly dies after sipping one of the cocktails being served, Cartwright believes it was murder, though Strange finds no poison in his glass. Sometime later, Poirot is in Monte Carlo and hears the news from Satterthwaite and Cartwright that Strange died from nicotine poisoning after drinking a glass of port wine, despite there being no trace in the glass. Except for the three men, Strange's guests are the same ones who attended Cartwright's party. Both Satterthwaite and Cartwright return to England to investigate the murders. They learn that prior to the party, Strange had sent his usual butler away for two months and that he exhibited strange behaviour as if expecting something. A temporary replacement he hired named Ellis has since disappeared, with Satterthwaite and Cartwright finding drafted blackmail letters from Ellis in his room. Babbington's body is soon exhumed, showing he also died from nicotine poisoning.

Cartwright, Satterthwaite, and Egg investigate the deaths, and Poirot joins them as a consultant. Each guest has a possible motive or suspicious circumstances surrounding Strange's death but no connection to Babbington. When Wills is interviewed, she recalls noticing Manders drop a newspaper cutting on nicotine and that Ellis had a birthmark on one hand; she later disappears. Poirot stages a party where he demonstrates how the murderer substituted the poisoned glasses while everyone's attention was on the victim. He then receives a telegram from Mrs De Rushbridger, a patient at Strange's Yorkshire sanatorium, who arrived on the day Strange died. Poirot and Satterthwaite go to meet her but find that she has, in turn, been murdered. Learning that Cartwright's servant, Miss Milray, is hastily heading to Cornwall, Poirot follows her to find out why.

Upon his return, Poirot assembles Cartwright, Satterthwaite and Egg, eventually denouncing Cartwright as the killer. Cartwright wants to marry Egg but already has a wife who resides in a lunatic asylum. As he could not divorce her under British law, he decided to conceal this knowledge by murdering Dr Strange, his oldest friend and the only one who knew about the marriage. After his party, Cartwright persuaded Strange to let him assume the role of his butler as a joke and then poisoned him during his party. He planted the nicotine cutting on Manders after tricking him into being at Strange's home. He falsified Ellis's blackmail letters, then travelled to Monte Carlo the day after to establish his alibi. The first murder was a dress rehearsal for the second to test whether the glass could be switched unseen, and the victim was selected at random. The only safe guests were Cartwright, Strange, who disliked cocktails, and Egg, to whom Cartwright gave a safe glass. Cartwright used Mrs De Rushbridger as a red herring to distract from Strange's behaviour towards "Ellis", and he killed her to divert suspicion and prevent her from revealing her ignorance of the case.

Poirot reveals that the nicotine came from rose spray distilled by Cartwright at an old tower near his Cornwall residence; the equipment was found by him when Miss Milray went to destroy it. His suspicions about Cartwright were based on several facts: Cartwright was the most likely to have poisoned the cocktail, his passport showed his return to England to play Ellis; Miss Milray's actions were motivated by a secret love for her employer; Miss Wills spotted Cartwright's similarity to Ellis and was spirited away by Poirot to protect her; and the telegram supposedly from Mrs De Rushbridger was addressed to Poirot when she knew nothing of his involvement. Cartwright flees, but Poirot says that he will "choose his exit" of public trial or suicide.

(In certain American editions, Poirot tells Cartwright that doctors and policemen await him in the next room. Cartwright, unable to believe someone as important as himself has failed, tries to prove Poirot a liar and is arrested when he opens the door.)

The shocked Egg is picked up by Manders, whom she initially cared for before Cartwright appeared. In the aftermath, Satterthwaite remarks how terrible it was that anyone, himself included, could have drunk the poisoned cocktail. Poirot remarks there was an even more horrendous possibility: "It might have been me."