Hercule Poirot's old friends Captain Hastings and Miss Lemon are astonished to learn from Assistant Commissioner Japp of Poirot's death! Grief-stricken, they assemble at the cemetery to bury their friend. Meanwhile, newspaper headlines scream "When Wil the Big Four Strike Next?" and in a Chinese-themed chamber, maniacal laughter echoes.
Four weeks earlier, a weasely journalist, Tysoe, aggressively questions foreign officer Ingles about the Big Four, a sinister, secret society bent on tipping the world into chaos. Ingles dismisses the cabal as utter nonsense—after all, he has real and dire concerns in the threat of war with Germany and Italy.
At a high-profile event sponsored by the Peace Party, an international organization devoted to preventing war, Poirot is reunited with Japp, who is there to provide security. Also in attendance: Madame Olivier (a prominent scientist studying the nervous system); diplomat and peace party supporter Steven Paynter; Paynter's personal physician, Dr. Quentin; and the blustery American tycoon and Peace Party funder, Abe Ryland. Their mentor, Li Chang Yen, is detained in China. The climax of the event is a chess match between reclusive chess master Savanaroff and Ryland. But mid-game, Savaranoff collapses, dead, apparently of natural causes. Tysoe sees the hand of the Big Four in Savaranoff's death, and when Poirot takes him up on his offer to learn more, Tysoe explains that Ryland made his fortune dealing arms to both sides in the war. Poirot declines his offer of information about the Big Four, attained from his secret source, in exchange for access to the investigation.
Remembering the Russian chess master's death grip on his chess piece and the blister on his fingertips, Poirot returns to the chess board on site and demonstrates for Japp and Ryland that using the chessman (which had been switched with the original and filled with a thin metal rod), Savaranoff instantly electrocuted himself when he placed it on the tampered-with board. Ryland makes a furious denial and Madame Olivier blames those who seek to discredit the Peace Party. But Ryland disappears.
Whalley, the reclusive author of the Li Chang Yen biography, is found dead in his home. The evidence points to his servant, Grant, but Poirot recognizes a shoddy frame-up and learns that Grant had been set up by someone first impersonating a priest from the Prisoner Help Society to get Grant the job in the house, then framed by the killer impersonating a butcher.
Throughout, an actress has been receiving anonymous notes backstage, professing love and a pending reunion. Then, Tysoe's informant is stabbed to death. Now spooked, Tysoe shares with Poirot the letters he'd received implicating Ryland as a member of the Big Four. He shows four playing cards found on the corpse, again implicating potential Big Four members: the monopoly tycoon (Ryland) a Chinese card (Yen), and the French Queen of Hearts (Olivier). The fourth card is the tarot "death" card.
Though dressed in quality evening wear, the informant was covered in ingrained dirt. A fragment of label remaining on his clothing reads "…erman." Meanwhile, at a dinner party, Diana overhears Paynter telling Olivier that he can't go through with something and concludes that they're having an affair. That night, she awakens to smoke drifting from under her husband's door. They find Paynter gruesomely burnt to death, his head sizzling on the electric heater. Poirot observes Paynter's fingertip stained blue, and from the bedside notepad, they find a large blue letter "G." Poirot again recognizes a crude attempt to frame a killer: it's been made to look as though Gerald, Paynter's no-good nephew, killed his uncle for revenge and fortune. He and Japp suspect Olivier, and Japp is convinced that the Peace Party is in fact working to start a war, rather than prevent one. But Madame Olivier disappears!
Poirot applies his considerable intellect to the case and finds, in the costumier's section of a theatrical trade directory, Max Berman & Sons. Forensics on Paynter reveals that he'd been drugged with Gelsemine—a drug firmly in Olivier's area of research—before his murder, further evidence that she is one of the Big Four. In the mysterious, Chinese-themed headquarters of, presumably, the Big Four, hooded figures, one Olivier and the other Ryland, sit as the disturbed laughter peals.
Returning to the author Whalley's house, Poirot asks about Whalley's estranged nephew, Albert, who hasn't returned to claim his inheritance. He looks through Albert's possessions and discovers a toy theatre and playbill after playbill, all from a now-defunct rep company called The Methuselah Players. Interviewing actors from the Methuselah, he finally lands on Flossie Monroe, the recipient of the anonymous love tokens. She shares her valentine with Poirot.
Poirot searches out an address and as he makes his way down to a flat an old lady shuffling down the street pauses then leaves. Poirot makes to enter the flat and then a massive explosion destroys the flat. His cane lies on the ground, charred, and his friends convene to mourn the dead detective. Together, they vow to figure out what Poirot knew and took to his death. Japp receives a telegram.
Meanwhile, Flossie shows up at the deserted, crumbling old Methuselah Theatre in response to a call for her to audition. As she searches the empty theatre, a mysterious person covers her mouth with a chloroformed pad, rendering her unconscious. When she comes to, she finds herself tied to a chair in the audience, a voice welcoming her to the lair of the Big Four. Onstage, Ryland and Olivier sit perfectly still in an elaborate Chinese set. Then Dr. Quentin emerges, but Flossie knew him as her besotted suitor from the Methuselah days, Claud Darrell, whom she'd rejected, telling him that she was meant to be with someone the whole world would remember. All this killing, all this intrigue, then, was for her. He unties her and is about to sweep her away when Poirot walks onstage: he lives!
As Japp arrives, Poirot unfolds the story, revealing that the Big Four are real only in the mind of one man: Albert Whalley, the orphan sent to live with his cold, aloof uncle who disapproved of this theatrical ambitions, who took the name Claud Darrell and later Dr. Quentin. He'd wormed his way into the confidences of the Peace Party players and played many roles: the priest greeting Grant from prison release; Ryland's chauffeur; the murderous butcher; Ryland's doctor. Ryland and Olivier were kidnapped, drugged, and kept in the theatre to throw suspicion on them. Whalley made Tysoe his pawn and planted clues on his informer—actually a tramp dressed in a theatrical costume. Savaranoff was killed to implicate Ryland, his uncle Whalley was killed to throw suspicion on Li Chang Yen, and Paynter was murdered to incriminate Olivier. All to impress the actress.
Poirot unraveled the mystery with the label of the costumier, the scrapbook of Methuselah productions, and Flossie Monro. When he got to her, he was getting too close, and that's when Whalley panicked. He called Poirot and arranged an interview, but Poirot noticed that the flat was suspiciously deserted and saw the old lady peering in. He tossed his cane inside the flat before it blew up. He then realized that he must commit to the charade of his death, so that Whalley – number four – would drop his guard.
Whalley steals Japp's gun but Tysoe releases the massive center stage curtain and it falls, crushing the mad killer.