Curtain: Poirot's Last Case is a 1975 novel by Agatha Christie, and the final story to feature the detective Hercule Poirot. It was published in the United Kingdom by the Collins Crime Club. The novel was written by Christie in the 1940s and locked away, as she was unsure of her own survival and wanted a proper conclusion for Poirot. She authorised its release shortly before her death.The novel returns to the same location as The Mysterious Affair at Styles, Poirot's first story. The elderly and crippled Poirot tells his old friend Arthur Hastings that one of the guests at the house is a serial killer, whom he calls "X", and he must act quickly to thwart the killer of their prey before it is too late...
Curtain contains examples of the following tropes:
Acquired Poison Immunity: Hercule Poirot drugs the murderer using his own sleeping pills, which he has been taking for many years. He uses the same gambit as Westley does in The Princess Bride, poisoning both cups while implying that only one cup is poisoned.
Adaptation Distillation: The Poirot adaptation (2013) mentions only the Litchfield, Sharples and Etherington murders, Margaret Litchfield is hanged during the opening credits (whereas she dies in an asylum in the novel), and the serial killer is not labeled as "X", although it is alluded when Poirot speaks to the others; but otherwise, it remains extremely faithful to the novel.
Dead Man Writing: Poirot's letter, sent to Captain Hastings four months after the former's death, reveals the identity of the serial killer labeled as "X" (Stephen Norton) and gives a reason why Poirot died, leaving the case seemingly unsolved until now.
Detective Mole: Poirot, sort of. He committed murder for the best of reasons though.
Face Death with Dignity: Poirot stops taking heart medication after killing Norton and accepts his heart condition in this way as atonement so that he can have a calm, dignified end with hours to spare.
Genteel Interbellum Setting: Averted, as Curtain actually does provide a timeframe for her stories (or at least the ones about Poirot, though this would probably drag a lot of others into the mix as well by proxy due to overlapping characters), placing them in the period of the early 1920s through the early 1940s. This may not always be consistent with the details of all of her stories but at least it's established.
He Who Fights Monsters: Defied by Poirot, who knew he possessed both the ability and the ego to become the very kind of serial killer he was always working to put behind bars. He orchestrates his own death, after having committed murder.
Karma Houdini: Averted by Poirot pumping lead into Norton's skull.
It's Personal: Hastings does not take Judith's flirtation with Allerton well. Norton exploits this to induce him to poison him.
The Last Dance: Poirot, dying of natural causes and dealing with a murderer he could not bring to justice through proof, kills the man himself and then accelerates his own death by withholding his heart medication.
Hastings: I was tired of this silly joking about my 'speaking countenance'. I could keep a secret as well as anyone. Poirot had always persisted in the humiliating belief that I am a transparent character and that anyone can read what is passing in my mind.
Meaningful Funeral: After Poirot has died of a heart condition, his funeral is arranged by his friend, Captain Arthur Hastings, and Hastings' daughter Judith, in which Poirot is laid to rest at Styles Court, which is the place where he lived when he moved from Belgium to England as a WWI refugee.
Miscarriage of Justice: This occurred repeatedly before the beginning of the story, thanks to X's repeated murders by proxy. Poirot manages to avert two further instances of this, firstly by drugging Hastings to stop him committing murder, and secondly by making sure that Mrs Franklin's death is reported as suicide. The latter instance is a case of choosing the lesser over the greater miscarriage of justice.
Murder by Mistake: Hastings turns around a bookcase, inadvertently swapping Mrs Franklin's coffee cup with a poisoned cup she had prepared for her husband.
Official Couple: Dr. Franklin and Judith by the end of the story. It is also hinted that Captain Hastings and Elizabeth Cole may become one as well.
Papa Wolf: Hastings. Goaded by Norton, he even plots murder in order to save his daughter.
The Perfect Crime: Even Poirot calls the murderer the perfect murderer, as he could never be tried, couldn't even be connected to the crimes, and gets away with over six murders (three in the TV adaptation). In fact, the only way to stop him was to kill him.
Poirot Grabs a Gun: Although Poirot has never shot or killed anyone until now, the fact that Stephen Nortoncan never be tried or connected to the murders that he gets away with puts the lives of the entire UK in danger, leaving it hanging in the balance while Poirot is dying of a heart condition; and he is pushed to the absolute limit so much that he has no other option but to shoot Norton dead in order to stop anymore crimes from happening. He could not say whether it was right to kill, but he is sure that it's for the benefit of everyone.
Pretty Little Headshots: Norton. The headshot turns out to be the clue to the identity of Norton's killer: Poirot himself whose legendary fastidiousness caused him to make an unnecessarily symmetrical headshot.
Rule of Symbolism: The result looks like the traditional interpretation of the brand of Cain.
Sadistic Choice: Norton's unprosecutable MO and lack of remorse pose one for Poirot.
Sissy Villain: Poirot speculates that it was Norton's resentment at his own "sissiness" and people's reactions to it that drove him to villainy.
Sympathetic Murderer: Poirot himself kills Stephen Norton, in order to prevent him from continuing his string of murders-by-proxy. A string which nearly turned Hastings into one of Norton's dupes. After killing Norton, Poirot lets himself die by not taking his medication.
Thanatos Gambit: Poirot neatly arranges his own death by putting his medicine out of reach, in order to atone for the murder he committed, leaving behind clues and a letter to Hastings.