A tray of cocktails is passed at a party in the clifftop deco mansion of Sir Charles Cartwright, the famous actor of the London stage. In addition to his old friends, Hercule Poirot and Sir Bartholomew Strange, Charles has invited the "May" of his aspiring May/December relationship, Miss "Egg" Lytton Gore, as well as a mix of glamorous Londoners and locals of modest means. Yet for the vicar, this party is to be his last; he suddenly drops dead after drinking his cocktail. While Egg cries foul and looks to Poirot to solve the murder, Poirot, with his gimlet eye, sees no evidence of foul play: the drinks were passed randomly, the glass shows no trace of poison, and the clergyman was a harmless old soul. But weeks later, Cartwright tracks down Poirot in Monte Carlo with the shocking news that Bartholomew Strange has died — poisoned under his own roof at a party he'd hosted for largely the same group of revelers. There is no longer any question: both deaths were, indeed, murders.
The party guests, or dramatis personae, as Sir Charles calls them, are not without their individual secrets, compulsions, and desires making them suitable suspects: the gambler, the high-end designer, the ambitious playwright, the penniless aristocrat, the rejected suitor and the faithful servant. With the enthusiastic Sir Charles and Egg on hand, Poirot strives to uncover a sinister plot that yields no trace of poison but a multitude of tantalizing clues. With a murderer on the loose, can the Belgian detective, a student of human character and motive, prevent the final exit of another player before the curtain closes on the third act?
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Warning: Contains significant plot spoilers
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Warning: Contains significant plot spoilers
Sir Charles Cartwright, the retired, renowned British stage actor, hosts his old friend Hercule Poirot at a dinner party at his Cornwall mansion. When one of the guests, the harmless old Reverend Babbington, chokes to death on his cocktail, Poirot dismisses murder because the drinks were passed randomly. This, he asserts, means that any one of Cartwright's guests might have taken the poisoned drink, had there been one (with the exception of his romantic interest, Miss "Egg" Lytton Gore, who was handed her drink, and his old friend Sir Bartholomew Strange, who drinks only wine). Therefore, neither victim nor motive can be established. Indeed, no trace of poison is later found in the glass. But Sir Charles is still convinced of murder, and watching his beloved Egg in an intimate conversation with his handsome young rival, Oliver Manders, he gloomily resolves to meet up with Poirot in Monte Carlo.
Yet in Monte Carlo weeks later, Poirot is greeted by Cartwright only to learn that Strange has died, this time indisputably of murder. At his Yorkshire estate, where he houses a sanatorium, the famous nerve doctor Strange had hosted nearly all of the same guests: the inquisitive playwright Miss Wills; the compulsive gambler Captain Dacres and his wife, the high-end designer Cynthia Dacres; and the penniless aristocrat Lady Mary, Egg's mother. Also joining the group, uninvited, was Oliver Manders, who showed up bloodied and injured after supposedly crashing his motorbike while passing by. Among his guests, about to make a revelation before the group, Strange choked to death on a glass of port.
Back in London, Poirot admits his mistake, vowing to solve the murders of Strange and Babbington. Sir Charles, ever the actor — and ever eager for an opportunity to impress Egg — asks if he may assist with the investigation. In Yorkshire, the duo meet with police Superintendent Crossfield, who confidently attributes the murder to Strange's new butler, Ellis, who vanished into thin air the morning after the murder. Crossfield learns from the coroner that Strange was poisoned with nicotine, a colorless, odorless liquid lethal at just a few drops. Yet the toxicology report reveals that the glass contained only port, with no trace of poison, just as with Reverend Babbington.
Poirot learns from the maid that Ellis seemed to share some sort of joke with Strange. She relates how when Ellis gave Strange a telephone message about a sanatorium patient named Mrs. Rushbridger, the doctor was surprised, questioning Ellis about whether he was sure, before jovially proclaiming Ellis's excellence as a butler. Poirot learns of a rumored secret tunnel leading from the house to the open countryside, perhaps the conduit of Ellis's escape. Surmising that Ellis was not what he seemed and connecting his odd relationship with Strange to the phone message about Mrs. Rushbridger, Sir Charles and Poirot venture to the sanatorium to inquire about the patient. But Mrs. Rushbridger has long been in no condition to receive visitors. Back at the estate in Ellis's room Egg, now joined in the sleuthing, discovers a splash of ink, which inspires Sir Charles to reenact the scene and find drafts of a letter of blackmail to the murderer. Poirot concludes that Ellis must be dead, another victim of the killer.
In London, Poirot is visited by Egg and the nervous Oliver, who admits that his motorbike crash was a fake. Strange had sent a letter requesting the stunt, but, Oliver explains, seemed genuinely surprised when he showed up.
Poirot and his amateur detectives Sir Charles and Egg separately question the other guests, learning that Captain Dacres bitterly resented Strange for fouling a deal which would have helped his wife's ailing business; that Miss Wills noticed a prominent birthmark on the butler's hand during dinner; and that Reverend Babbington had warned Egg away from Oliver.
Now convinced of greater danger, Poirot hosts a sherry party, reuniting the surviving guests and staging a dramatic poisoning of Sir Charles to demonstrate the murderer's untraceable method: switching a clean glass for the deadly one when everyone's attention is fixed on the victim. His other goal is to note the expression on one face in particular when Sir Charles "dies." But a real death is soon learned of: Mrs. Rushbridger was poisoned with an anonymous gift of chocolates, only shortly after sending a telegram to Poirot stating that she had information about Strange's death.
Sir Charles jokes about his given name, "Mugg", proposes to Egg and is accepted. But their joy is tempered by Miss Wills's disappearance, even as her newest play enters dress rehearsals. There, Poirot theatrically unites all the revelers onstage to reveal the murderer: not the Dacres, because they had never known Reverend Babbington, not Miss Wills, who actually joins the group onstage, and not Ellis, because he never existed. It was Sir Charles who created the character Ellis with Strange's joking complicity; who then traveled to Monte Carlo and met Poirot; who forged a letter to Oliver to place him at and incriminate him in the murder; and who forged blackmail letters then hid them in Ellis's room. Sir Charles then introduced Mrs. Rushbridger into the mystery to draw attention away from Ellis — especially after Sir Charles noticed Miss Wills connecting the actor's hands to those of the butler. Aware of Miss Wills's suspicion, he forged a telegram from the sanatorium patient then poisoned her, all to obfuscate and distract from looking too closely at Ellis.
But only upon finally realizing a motive could Poirot truly indict Sir Charles: before changing his name from Mugg to Cartwright, he was married — and remains married to a woman he cannot lawfully divorce, a patient at an insane asylum. Unable to propose to Egg as long as the one other person who knew of the marriage was alive, his best man, Strange, he resolved to clear his path by killing his old friend. And the Reverend Babbington? His murder was no more than the great actor's dress rehearsal — any guest other than Sir Charles's beloved Egg or his next victim Strange might have taken the glass and perished that night...even his old friend, Poirot.