Plot
In 1947, Hercule Poirot lives in retirement in post-war Venice, employing ex-police officer Vitale as a bodyguard. On Halloween, mystery writer Ariadne Oliver convinces Poirot to attend a séance at the palazzo of opera singer Rowena and expose medium Joyce Reynolds as a fraud. Rowena has hired Joyce to help her commune with her daughter Alicia, who reportedly committed suicide after her fiancé, chef Maxime, broke off their engagement. Among the guests in attendance are Maxime, Rowena's housekeeper Olga, Drake family doctor Leslie and his son Leopold, and Joyce's Romani assistant, Desdemona Holland.
During the séance, Poirot deduces that Joyce has two assistants, revealing Desdemona's half-brother Nicholas hiding in the chimney. Joyce suddenly speaks in Alicia's voice and reveals she was murdered by one of the guests. Poirot attempts to confront Joyce, who tells him to lighten up and gives him her mask and robe. Seconds after an unknown assailant nearly drowns Poirot when he is apple bobbing, Joyce is found impaled on a statue in the courtyard.
With a storm cutting off the palazzo, Poirot interviews the guests, during which he witnesses manifestations of Alicia's ghost and hears a young girl humming a tune. The investigation yields perplexing results:
The severely traumatized war veteran Leslie is in love with Rowena.
Maxime, who was not initially invited, broke off his engagement because Rowena did not approve of him, and Alicia was obsessed with keeping her mother happy.
Leopold claims to hear voices from the spirits of children left to die of plague back when the palazzo was an orphanage, as well as the voice Poirot has been hearing. The spirits are said to haunt and kill any doctors or nurses who set foot within it.
Joyce had served as a nurse during World War I.
Nicholas and Desdemona have been stealing from Joyce and intend to use the money to travel to St. Louis, Missouri. Nicholas claims his showmanship was in support of Joyce's genuine visions, but Desdemona says it was all fake.
When the guests come across an underground chamber containing the skeletal remains of the dead children, Leslie suffers a panic attack and nearly kills Maxime. He is locked inside the music room to recover, Rowena giving Poirot the only key. After examining Maxime's invitation, Poirot deduces Oliver sent it and is conspiring with Vitale. Vitale, who investigated Alicia's death, gave Joyce private details, while Oliver had hoped to use Poirot's incapability of explaining the supernatural as a plot for her next book. Leslie is then found stabbed to death.
Gathering the remaining guests together, Poirot reveals Rowena is the murderer. Obsessed with keeping Alicia for herself, she cut her off from contact with Maxime after learning they planned to reconcile and used small amounts of poisonous honey to weaken and then care for her. One night, Olga unknowingly gave Alicia tea containing a fatal dose of the honey. Rowena, fearful of exposure, staged Alicia's suicide. When she began receiving blackmail threats, Rowena suspected either Joyce or Leslie. She pushed Joyce to her death after mistakenly attempting to drown Poirot and forced Leslie into stabbing himself via the palazzo's internal phone line by threatening to kill Leopold, hoping to pass off both deaths as part of the children's curse. When Poirot confronts Rowena on the roof, Alicia's ghost seems to appear to them both, pulling Rowena down off the building to her death.
As dawn breaks, Poirot ends his friendship with Ariadne, elects not to unmask Vitale's involvement in the séance, keeping him on as a bodyguard, and privately exposes Leopold as the blackmailer. Leopold explains he understood the poisoning signs his father missed, making the connection after realizing Rowena's first starring role was in an opera whose lead character is the "king of poisons". Poirot suggests Leopold and Olga clear their consciences by using the money to help the Holland siblings to start a new life in America. After noting the possibility of an afterlife, he then returns home to accept new cases.
