In post-war Venice, the retired detective Hercule Poirot enjoys his peaceful retirement while employing a bodyguard, Vitale Portfoglio. On Halloween, the renowned mystery writer Ariadne Oliver persuades Poirot to attend a séance at the opulent palazzo of opera diva Rowena Drake. This is intended to unmask Joyce Reynolds—a purported medium.
Rowena's motive for employing Joyce is to make contact with her late daughter Alicia. She seemingly took her own life after her relationship with Maxime Gerard ended tragically. During the séance, Poirot astutely discerns that Joyce has more than one assistant eventually revealing a hidden figure named Nicholas. However, their pursuit of truth takes an ominous turn when Joyce is killed and impaled on a courtyard statue.
Poirot begins to suspect that Ariadne Oliver might have been collaborating with Vitale for her literary endeavors. Meanwhile, Leopold reveals to hear the voices of long-deceased children from when the palazzo was an orphanage. Further disturbing revelations become known when an underground chamber is discovered with the skeletal remains of the orphaned children. This shocking discovery triggers a sequence of events leading to Leslie's death, which prompts Poirot to gather the guests for a final revelation.
Poirot uncovers the true murderer, Rowena Drake, as her obsession with keeping Alicia for herself makes her poison her daughter with hallucinogenic honey. Alicia's suicide was staged, and blackmail threats led Rowena to eliminate Joyce and Leslie.
In a chilling rooftop confrontation, Poirot confronts Rowena but an eerie intervention by what appears to be Alicia's ghost occurs. It causes Rowena's demise as she plunges into the canal waters. As the dawn breaks, Poirot makes critical decisions that alter his relationships with his associates.
